This summary is not the rule. The rule text controls.
When someone testifies in court, the other side can question whether that person is telling the truth. One way to do this is by showing the witness might be biased or has something to gain or lose based on the outcome of the case.
For example, if a witness is a close friend or relative of one side, that connection might affect how honestly they testify. The same goes for someone who stands to make money, avoid trouble, or get some other benefit depending on which way the case turns out. Lawyers are allowed to bring up these kinds of relationships or motives so the judge or jury can decide how much to trust what the witness says.
This rule matters because it gives both sides a fair chance to point out reasons a witness might shade the truth, whether on purpose or without realizing it. It does not mean the witness is lying. It just means the jury gets to hear the full picture and weigh the witness's word accordingly.