In the American legal system "queen for a day" refers to written documents that are designed to create a potential mutually beneficial arrangement with the federal government and a person of interest regarding a criminal investigation. The concept of a "queen for a day", more commonly known as proffer agreements permit the accused individual to disclose to authorities key points of knowledge to crimes committed by that person and/or others, with implied assurance that said knowledge will not be used against them in later proceedings.
More precisely it facilitates a symbiotic relationship by giving prosecutors a sample of the individual's knowledge that in turn provides the accused leverage to bargain with for something such as a lesser sentence or immunity in regards to the whole of the investigation.
I'm just glad I had enough shit to feed the feds on everyone else for that robbery we pulled.
If it wasn't for my "queen for a day" deal with them I'd be doing 50 years instead of 5.