In the context of criminal law, a proffer agreement is a written agreement between federal prosecutors and individuals under criminal investigation which permit these individuals to give the government information about crimes with some assurances that they will be protected against prosecution. Witnesses, subjects or targets of a federal investigation are usually parties to such agreements.
Proffer agreements are not complete immunity agreements. Although the government cannot use actual proffer session statements against the individual in its case-in-chief, the information provided can be used to follow up leads and conduct further investigations. If those leads and further investigations lead to new evidence, the new evidence can be used to indict and convict the individual who gave the information in the proffer session.
Proffer agreements typically contain a clause allowing the government to impeach you with your proffer statement in a subsequent proceeding if your testimony is inconsistent with your proffer.