This provision is as a practical matter obsolete. It is providing for what is known as "apportionment" of a direct tax. If State A has a population that is 10% of the total population of the US as a whole, according to the latest census, then any direct tax must be so structured that 10% of the revenue comes from state A. This doesn't work very well for anything except a "head tax" -- that is each person pays a fixed amount, say $10 per person.
This was designed to eliminate or at least reduce the motive to cheat on the census, because while a higher number got a state more representatives in Congress, it also increased the direct tax burden. But apportionment proved do awkward that Congress effectively never passed any direct taxes.
When the Supreme Court held that an income tax was a direct tax, the income tax was simply abolished until a constitutional amendment exempted it from apportionment.
So far as I know, no current Federal tax is subject to apportionment.
The 3/5ths provision included here referred to slaves, who were counted as 3/5ths of a person for this purpose. It has been obsolete since the 13th amendment outlawing slavery.