Speculation is a type of objection that a lawyer may use during a trial. In such an objection, the lawyer would be trying to claim that the witness has no way of knowing the answer, and that the opposing counsel is asking them to guess the answer.
If a judge agrees that speculation is occurring, they will likely side with the lawyer who objected.
The reason why speculation can be objected to in court is because witnesses are not supposed to guess the answers to questions; doing so can create confusion for the jurors.
For example, if a lawyer asks a witness why a defendant committed a crime, and the witness had never talked to the defendant in their life, this would be speculation. If the witness had to answer the question, they would have to make a guess, which would not be accurate information.