While there are a few different meanings to this term, the main use of it has to do with the questioning of prospective jurors by attorneys before a trial. This process is used to pick a jury that will hear the case.
Voir dire is sometimes used to refer to the process of ensuring that a witness has the proper qualifications. For example, if a person is being put on the stand as an alleged expert psychiatrist, they may be questioned to establish that they really are an expert. This is a form of voir dire.
However, the term is much more often used to refer to the process of questioning prospective jurors. At the beginning of a trial, there is a large panel of people from whom the jury may be selected. Lawyers for each side are allowed to question them to determine if they would be good jurors (from that attorney’s point of view).
The lawyers ask prospective jurors questions. If the answers show bias, the attorneys may ask the judge to strike the juror for cause. For example, imagine a capital murder case with a female defendant. If a potential juror says they do not believe women should be executed, they might be struck because they would be biased.
The jurors also have peremptory challenges that they can use to get rid of prospective jurors that they simply do not think would be good for their side. For example, an attorney defending a young person accused of vandalism for writing graffiti might want to remove jurors who look like buttoned-down conservative people because they would be too likely to convict.
This is the voir dire process.