It's the whole truth of your experience
If I were to say "I saw John going into the store", in reality it may have been someone else, or they may have simply walked past the store (if you saw them from an angle for that to be possible). Or maybe you were just hallucinating or misremembering.
But it is a true account of (your memory of) what you experienced.
If you want to be more exact, you could also say "I saw someone who I believe to be John going into the store. Well, I can't be entirely sure they actually went into the store given the angle from which I saw it, but I can't imagine them having done anything else." Of course if you add that much doubt to your own statement, you'd probably hurt the side your testimony is meant to help, whomever put you on the stand probably won't be happy with you and it may lead to a verdict that doesn't reflect the truth. That may however be acceptable to the court, and it may even be recommended or necessary in cases where you're just not sure enough (but perhaps phrased a bit better).
If you're asked for a yes/no answer you can't be 100% sure of, you can (and should) clarify that you're not 100% sure (and give the answer you believe to be true, if any). If nothing else, you can always say something like "yes, that is what I believe to be true". If the judge insists that you answer just yes or no despite this, then you should probably do what they say if able. More than anything else, refusing to do what the judge says is how you end up being charged with contempt. If you have already clarified that you're not absolutely sure, there's no need to keep stating this for the same question. Your objection may also result in them adding something to the effect of "to the best of your knowledge" to the question, which should address your concerns.
If you're asked about the state of the world, for example, like where someone else was at some point in time, the same applies. If you're asked "Where was John during all of this?", you can say "I believe he was..." or "He told me he was going to..." or whatever makes sense. Unless you were looking at him at that exact time, you can't be 100% sure where he was, so it's reasonable to not just give an objective answer like "He was...".
It's implied to not be the objective truth
It being your experience, or a conclusion based on the knowledge you have available, is also generally implied.
If you say "I was outside the store; John went inside" or "I saw John get out of his car; he then went into the store", the implication is that you saw John going inside. It is not necessary to explicitly specify that.
Although there may be cases where it would make sense to explicitly specify how you know what you're stating. Perhaps you heard, rather than saw, him go inside. Perhaps you looked away and then he was gone, with the only place he could've gone being inside. That could affect how trustworthy your testimony is.
The above are just intended to be examples to illustrate my point, not recommendations for what you should say if you ever find yourself on the stand. There is a lot of nuance here. A lot may be implied based on how you phrase your answer. For recommendations on what you should say, I would recommend consulting your lawyer.
There is also perhaps the philosophical question of how trustworthy our own experiences are and how sure we can really be of the true state of the world based on that. But that's definitely not something you should be bringing up when on the stand.