tl;dr means Too Long; Didn't Read. Basically it's a summary of whatever the user wrote. If it's a story of a kid making fun of her breasts and then getting his head stuck in a deer's ass, you could read the whole story or just the summary.

NSFW means not safe for work. It's refers to nudity or gore or something horrible in the gif or the picture or whatever. It's something you shouldn't see at work because you could get in trouble. NSFW is also used in some subreddits to warn of spoilers for a tv show or a movie.

MFW means "my face when. . ." It's a reaction to something. My face when I missed my first cake day

MFW I'm flirting