A more thorough explanation:

The state-of-mind exception is a legal principle that allows an out-of-court statement about a person's existing motive to be admissible as evidence, even if the person who made the statement cannot testify in person. This exception is an exemption to the general rule that hearsay is not admissible in court.

For example, if a person writes in their diary that they plan to harm someone, and that person is later found dead, the diary entry may be admissible in court as evidence of the person's motive to commit the crime.

Another example is if a person tells a friend that they are afraid of someone and believe that person may harm them, and then later that person is found dead, the friend's testimony about the statement may be admissible as evidence of the person's state of mind and fear of the other individual.

These examples illustrate how the state-of-mind exception allows certain out-of-court statements to be used as evidence to prove a person's motive or state of mind at the time of a crime. This exception is important in cases where the person who made the statement is unavailable to testify in person, but their statement is still relevant to the case.