Often used for "for your information", in many cases this acronym more appropriately conveys "fuck you, idiot!"
Boss: Joe, you did finish your report by the deadline this time, didn't you?
Joe: FYI, I finished it a day early!
Wife: Honey, I thought I asked you to mow the yard yesterday?
Husband: FYI, I ran over my foot with the lawnmower so I thought I'd wait to get my prosthesis before I finished the job.