Let's say I have a doctor on the witness stand. He's my medical expert. I want to ask him questions that discuss a particular study that will support our position.
"Dr. Jones, please explain to the jury what this study did and why it supports our claim..."
"Objection!" yells the defense lawyer.
"What's the objection?" asks the trial judge.
The defense lawyer then gives a long-winded legal answer to the judge's question.
"Your honor, I'd like to make an offer of proof outside the presence of the jury..." I say to the Court.
After the jury is excused I will now tell the judge why this study is important. I will ask the doctor questions designed to explain to the judge why it is relevant and important to our case.
An offer of proof is a sampling and preview of what the witness will testify about and tell the jury.
The judge will then have to decide whether to allow the doctor to answer questions about this study or whatever it is that we're trying to prove.