First, when an attorney makes an objection during trial that means he does not like the way a question was asked or believes it's an improper question. He then must state the reason succinctly to the judge. The judge must then decide whether to allow the witness to answer the question or not.
When the attorney claims that the question assumes facts not in evidence, what he is really saying is that the facts that are being presented to the witness are presumably not yet in evidence and therefore, how can this witness properly answer the question if those facts have not been put before this jury?
This type of objection usually arises when an attorney asks a witness a hypothetical question or when certain testimony has not yet come into evidence because a certain witness or witnesses have not yet testified, but will.
If there is a question about whether certain factual information has already been given at trial, the judge may allow the witness to answer and claim that the jury's recollection will govern about whether or not those sets of facts are already in evidence.
For example, if the attorney asks a witness to assume that certain facts are true, and then asks for an opinion about those facts, if those facts are not yet in evidence, the witness may still be allowed to answer the question with the expectation that that information will come in at some later point in the trial.
What happens though if those factual pieces of evidence never come into evidence? In all likelihood, the judge will give the jury an instruction that basically says if you find that those facts that have come into evidence are true, then you are permitted to give that witness' testimony whatever weight you feel it deserves.
If however you feel that those facts have not come into evidence or that they are not true, you are permitted to disregard that witness's testimony on those points.
The attorney who is asking the question will likely argue that those facts are in evidence already or they will be coming in shortly with other evidence and other witnesses.
