A proffer agreement allows a person to describe what they know to prosecutors without those statements being used directly against them in the government’s case-in-chief. It is the usual precondition to any cooperation discussion.
The protection is narrower than it sounds, and the exceptions are the point. Typically the government may use proffered statements to derive leads and pursue evidence from other sources; to cross-examine if the person testifies inconsistently at trial; and to rebut evidence or arguments the defence offers that contradict the proffer. The last exception is the widest — it can substantially constrain the defence a person may run later, because contesting facts they admitted in the proffer opens the door to it.
There is also no protection against a false statement charge for lying in the proffer itself.
The decision to proffer is therefore not a low-cost information exchange. It is a significant step that narrows future options in exchange for the possibility of a cooperation agreement, and it should follow rather than precede a careful assessment of the exposure and the strength of the government’s existing case.