Hearsay is a statement or some conduct that is similar to a statement or assertion that is made out of the court and is offered into evidence as truth. For example, when a police officer wants to obtain an arrest warrant he may offer the judge issuing the warrant a statement made by a confidential informant who cannot be present and is a reliable source of information.
The admissbility of these statements is governed by what is called the hearsay rule, which prevents testifying witnesses from stating that either an absent individual said something that will prejudice the outcome of the case so it prevents from statements like "he said..." or "she said..." there are many exceptions to the hearsay rule and thus out-of-court statements will be allowed into evidence. The judge has a lot of leverage on determining whether a piece of hearsay evidence will be admissible or not.
Having an Experienced Criminal Defense Attorney on your side that understands these rules can be crucial to your case.