This term is related to admissible and inadmissible evidence.  There is a general rule that quotes from a witness not present in the court are inadmissible forms of evidence.  The main reason for this is that the witness cannot be there in person to be cross-examined by the opposing lawyer.  There are usually follow up questions that can be important to the decision in the case.  Additionally, the jury is not able to judge the witness at face value when they are giving their testimony.

However, there are exceptions to this, in which testimony or evidence can come form a third party not present in the courtroom.  The main requirement for this is that the evidence be spontaneous, reliable, or from an official deposition.  There are also other stipulations and special circumstances that are up to the trial judge to decide upon whether an evidence falls under the hearsay rule.

Example

In a lawsuit brought about from a Guam Estate trial case, one son accuses another of influencing the father in his dying days to modify the will.  Emails are presented as evidence that show that the older brother apparently sent emails to his acquaintance telling him of his plans to persuade the father.  This was spontaneous, and showed the mindset of the accused at the time.  Also, the email service verified the authenticity of the email.  Because the evidence was deemed both reliable and spontaneous, the trial judge found it not to fall under the hearsay rule, thus making it admissible evidence.