Working with or collaborating with law enforcement. Indicates someone is “wearing a wire”, but not in a literal sense. Snitching, passing information to police.
Robin: Let's make a pact, if we both turn 40 and we're both single..
Ted: Robin Scherbatsky, will you be my Back-Up Wife?
Robin: A girl always dreams of hearing those words. Yes, yes, a million times, yes!