Rule 401 As used in this article, “proffered evidence” means evidence, the admissibility or inadmissibility of which is dependent upon the existence or nonexis…

When someone wants to use a piece of evidence in court, a judge sometimes has to decide if certain background facts are true before allowing that evidence in. "Proffered evidence" is just the legal term for that piece of evidence waiting to be approved or rejected by the judge.

For example, if someone wants to use a recorded phone call as evidence, the judge might first need to confirm that the voice on the recording actually belongs to the person being accused. The recording is the "proffered evidence," and proving whose voice it is would be the background fact the judge needs to check first.