A more thorough explanation:

An immaterial fact is a fact that is not relevant to a matter in issue. It does not affect the outcome of a case or dispute. For example, if someone is accused of stealing a car, the color of their shirt on the day of the alleged theft is immaterial because it has no bearing on whether or not they committed the crime.

Another example of an immaterial fact could be the weather on the day of a car accident. While it may be interesting to know if it was raining or sunny, it does not impact who was at fault for the accident.