A more thorough explanation:

An offer of proof is a response made by a lawyer when the opposing counsel objects to the admissibility of evidence during a trial. If a lawyer introduces evidence, either in the form of an exhibit or witness testimony, and the opposing counsel objects to its admissibility, the proponent of the evidence has the chance to respond to the objection with an offer of proof.

The purpose of an offer of proof is twofold. Firstly, it provides the proponent of the evidence with an opportunity to persuade the judge not to exclude the evidence. Secondly, it preserves the error on the record for appellate review.

When making an offer of proof, a lawyer's goal is to describe the evidence, explain the purpose of introducing the evidence, state the grounds for admissibility, and sufficiently inform the appeals court of the consequences of excluding the evidence.

For example, if a lawyer is trying to introduce a witness's testimony about a conversation they had with the defendant, and the opposing counsel objects on the grounds of hearsay, the proponent of the evidence can make an offer of proof. The offer of proof would explain the relevance of the conversation to the case, the reliability of the witness, and the exceptions to the hearsay rule that make the evidence admissible.

In federal court, an appellate court cannot set aside or reverse the verdict of the trial court if an offer of proof is not provided at trial unless the purpose of the evidence is apparent from the context. This means that it is essential for lawyers to make an offer of proof to ensure that the evidence is properly considered and that any errors are preserved for appeal.