A more thorough explanation:

Definition: Double hearsay is a type of hearsay statement that contains further hearsay statements within it. This means that the statement is based on what someone else said, who in turn heard it from someone else. Double hearsay is generally inadmissible in court unless exceptions to the rule against hearsay can be applied to each level of the statement.

Example: Let's say that John tells Mary that he heard from his friend Tom that Tom's neighbor, Amy, admitted to running a red light. In this case, John's statement to Mary is hearsay because he did not witness Amy admitting to running the red light himself. However, John's statement also contains hearsay within it because he is relaying what Tom said about Amy's admission. This is an example of double hearsay.

The example illustrates how double hearsay can occur when a statement is based on what someone else said, who in turn heard it from someone else. In this case, the statement is not based on the personal knowledge of the witness, but on the credibility of others. Double hearsay is generally not admissible in court because it can be unreliable and difficult to verify.