Offer of Proof
An offer of proof is explaining why something should be admitted or not. To make an offer of proof, just ask "May we approach?"
Offers of proof are governed by FRE 103.
An offer of proof can be made by narrative or by witness testimony.
By narrative is the most common way and is just making the argument.
By testimony requires the jury to leave the room so the witness can be questioned, so it is often not done.
At sidebar, the jury will still be in the room, so you must be very quiet.
They're often for the record for appellate courts.