A more thorough explanation:

Definition: The principle that a witness should testify to facts, not opinions, and that a nonexpert witness's opinions are often not allowed as evidence.

Traditionally, this principle is considered an important rule in evidence law. It is based on the idea that a witness who has observed data should provide the most factual evidence possible, leaving the jury to draw inferences and conclusions from the evidence. Under this system, the witness's opinion is unnecessary. Today, opinions are admissible if they are based on a witness's perceptions and helpful to the fact-finder.

Example: A witness in a car accident case can testify that they saw a car run a red light and hit another car. This is a fact that they observed. However, if the witness says that the driver of the car that ran the red light was reckless, this is an opinion and may not be allowed as evidence.

This example illustrates the opinion rule because the witness is providing an opinion about the driver's behavior, which is not a fact that they observed. The jury can draw their own conclusions about the driver's behavior based on the facts presented.

Example: In a murder trial, a witness who is a medical examiner can testify about the cause of death based on their examination of the body. This is an opinion, but it is based on their expertise and is helpful to the fact-finder.

This example illustrates that opinions can be admissible if they are based on a witness's expertise and are helpful to the fact-finder. The medical examiner's opinion about the cause of death is important evidence in the case.