According to former Special Prosecutor Sol Weisenberg:
Just what is a proffer and what are the perils of entering into a proffer agreement (also known as a proffer letter) with the federal government? Proffer or "queen for a day" letters are written agreements between federal prosecutors and individuals under criminal investigation which permit these individuals to tell the government about their knowledge of crimes, with the supposed assurance that their words will not be used against them in any later proceedings. (The individuals can either be witnesses, subjects or targets of a federal investigation, although it is subjects and targets who provide most proffers.)
If you enter into one of these agreements, you will proffer information orally in a proffer or queen for a day "session" attended by you, your attorney, the Assistant U.S. Attorney ("AUSA") and one or more federal agents. (In recent times, regulatory attorneys have been attending proffer sessions, when the government is engaged in parallel civil and criminal investigations.) You should think of a proffer session as a sneak preview in which you show the federal authorities what you can bring to the table if they cut a deal with you.
A proffer is a statement that you make when being charged with a crime in Federal Court. It is usually given at a time when you are considering taking a deal and/or cooperating with the U.S. Attorney's Office and they want to confirm how much you know. The prosecutors can also offer a proffer regarding evidence they think they will present before the Federal Court. It is often a tool used to determine whether or not a party as sufficient information to offer somethign as evidence, and/or to determine whether or not a plea should be given.