Definition - What does Relevance mean?
Relevance is the degree to which something is related to the evidence that is brought forth in a trial.
If an attorney believes that the opposing counsel or a witness is discussing something that is not relevant, then they can object. If the objection is approved, then the irrelevant words can be struck from the record.
Justipedia explains Relevance
Often, attorneys will object if they think that a witness or the opposing counsel is discussing something that has no connection to the trial. For example, if the trial is about fraud, and the prosecution is discussing the defendant's history of driving while under the influence, the defense attorney may object over relevance. Unless the prosecutor can explain how the two things relate to one another, the judge may declare that the comments should be struck from the record.