A more thorough explanation:

Definition: Collateral matter refers to any evidence that could not have been introduced for a relevant purpose. For example, if a witness has made an error in testifying about a detail that is not relevant to the facts of the case, then another party cannot call witnesses to contradict that point. Cross-examination alone must suffice.

Example: During a trial, a witness testifies that they saw the defendant wearing a blue shirt on the day of the crime. The color of the shirt is not relevant to the case, but the fact that the witness saw the defendant is. If the defense tries to call a witness to contradict the color of the shirt, the judge may rule that it is a collateral matter and not allow the evidence to be presented.

Explanation: The example illustrates how collateral matter is any evidence that is not relevant to the case. The color of the shirt is not important to the case, so it is considered collateral. The judge may not allow the defense to present evidence about the color of the shirt because it is not relevant to the case.