What does this paragraph mean? Line by line.
I give my Residuary Estate
This is a gift, effective when the person writing the Will dies, of everything that is left over after all debts and taxes are paid and after any other gifts already in the Will (e.g. leaving a car or a house or a Monet to someone in particular) have been given.
to the said [Full Name] absolutely
I'm going give [Full Name] a name so that it is easier to follow this explanation. [Full Name]'s name for purposes of this answer is "Luna".
This says to give all that stuff to Luna when the person who wrote the Will dies, if Luna is still alive for whatever the required amount of time is after the person who wrote the Will dies.
The required survival time period is either in the boilerplate provisions of the Will, or in the relevant statute if the Will is silent on the question.
The Will says "absolutely" because historically, someone who received gifts of property in deed or wills in England only got to keep it for their lifetime, unless it clearly specified otherwise, after which someone else would get it. But in this case, if Luna survives this long, Luna gets all of this stuff with no strings attached.
and if [Name] shall fail to obtain a vested interest leaving issue who
survive me then such issue shall take by substitution
If Luna dies before the person who wrote the Will does, or doesn't stay alive for the required number of days afterwards, then Luna isn't entitled to this stuff. Luna's descendants get it instead (i.e. Luna's descendants "take by substitution" what Luna would have gotten if Luna had lived, instead of Luna's probate estate getting the stuff).
and if there shall be more than one of such issue they shall take in
equal shares per stirpes but so that no issue shall take whose parent
is alive and so capable of taking.
If Luna predeceases and has exactly one living descendant who is alive when the person who wrote the Will dies, and that living descendant lives the required number of days after the person who wrote the Will dies, then the sole living descendant of Luna gets all of the stuff that is left over when the person who wrote the Will dies.
If Luna has more than one living descendant, the stuff that is left over when the person who wrote the Will dies, then Luna's descendants gets broken up the way described below, which is called per stirpes:
Create one share for each child of Luna who is alive and survives Luna by the required amount of time. If Luna has only one living child, that child gets everything even if Luna's child has children of their own.
Create one share for each child of Luna who didn't live for long enough after the person who wrote the Will's death, if the predeceased child has descendants who are alive and remain alive after the person who wrote the Will dies by the required amount of time. This share is then broken up into one sub-share for each child of the predeceased child who is alive when the person who wrote the Will dies and is still alive after the person who wrote the Will dies by the required amount of time, and one sub-share for each predeceased child of the predeceased child who has living descendants who remain alive for the requisite number of days.
Continue this process until 100% of the the residuary estate has been assigned to someone and give them their share of it once the estate is settled.
If someone lives past the minimum number of days to outlive the person who wrote the Will, and then dies, that person's share goes that person's probate estate.
For visual learners, a per stirpes distributions of assets looks like this:

A per stirpes distribution to descendants is the plain vanilla ordinary way to giving stuff to the descendants of a dead person when you don't know in advance who will outlive you.
What if Luna predeceases with no living descendants?
Usually, this paragraph of a Will will be followed by another paragraph called the "ultimate contingent beneficiary" which says who gets the stuff that's left over in the residuary estate if Luna predeceases the person who wrote the Will and has no living descendants.
Often, the ultimate contingent beneficiary will be one or more distant relatives, a list of friends, or a charity.
If there is no ultimately contingent beneficiary in the Will, but Luna and all of Luna's descendants predecease the person who wrote the Will, then it goes to the next of kin (a.k.a. "heirs at law") of the person who wrote the Will, if there are any relatives of the person who wrote the Will who are close enough to qualify to inherit under English inheritances law.
If there is no one closely enough related to the person who wrote the Will to qualify under English inheritance law, then the stuff "escheats" (i.e. is inherited by default) by the King (or Queen) of England, as the case may be.
There are some circumstances when the Will can be ignored.
Everything above explains what this language in the Will means. This isn't always what happens, however.
There are several exceptions to the general rule that property goes to the people that the Will says it goes to. I won't list them all here, but it is important when a Will is being written to understand that this is the case.
For example, if the person who wrote the Will leaves nothing in the Will to their spouse of thirty years as of the death of the person who wrote the Will, who has no assets of their own, then the Court will partially ignore what the Will says and give some of the residuary estate to the surviving spouse.
Also, the Will only controls assets that are in the "probate estate". Some assets pass at death in what are called "non-probate transfers" that are not controlled by what the Will says.
And, finally, of course, if the Will was written when the person signing the Will was of unsound mind, the Will can be invalidated in a "Will contest" in the appropriate court if the person contesting the Will's validity can prove that the person signing it was of unsound mind at the time.
This is mediocre legal writing
This paragraph gets the job done, but it is not very well written by modern legal Will drafting standards.
It is adequate and probably meets the standard of care for a lawyer who hasn't committed malpractice. Lawyers in England have been writing paragraphs like this one for three or four hundred years.
But it is not "best practices" legal writing in a Will. Good modern legal drafting for a Will would be much easier for a non-lawyer to understand, in addition to being clear and unambiguous.