A more thorough explanation:

Definition: Hearsay within hearsay refers to a statement that contains further hearsay statements within it. This means that the original statement is based on what someone else said, and that person's statement is also based on what someone else said. Both statements may be inadmissible in court unless exceptions to the rule against hearsay can be applied to each level.

Example: Imagine that a witness testifies in court that they heard their friend say that they saw a man steal a purse. This is already hearsay because the witness did not see the theft themselves. However, if the friend's statement also includes information from someone else, such as "my neighbor told me they saw the man run away," then this is hearsay within hearsay. Both the friend's statement and the neighbor's statement may be inadmissible unless exceptions to the hearsay rule can be applied to each level.

Another example: A police officer testifies that they heard a dispatcher say over the radio that a witness reported seeing a red car speeding away from the scene of a crime. This is hearsay because the officer did not witness the witness's report themselves. However, if the dispatcher's statement also includes information from the witness, such as "the witness said they saw a man wearing a blue shirt driving the car," then this is hearsay within hearsay. Both the dispatcher's statement and the witness's statement may be inadmissible unless exceptions to the hearsay rule can be applied to each level.