It's informal and you would usually use it via text messaging. It means 'talk to you later'
Short for "Talk To You Later".
commonly used on massively multiplayer online games and chat rooms.
Dood1: Hey man i g2g eat, TTYL!
Dood2: Sure bud, bonapetit!
A abbreviation of the sentence talk to you later mostly used in text messages between people most teens use it.
Teen 1- Ok i'll ttyl
Teen 2- Ok bye :-)
TTYL means Talk To You Later. Mostly used in text conversations.
TTYL bye
Frank:''That was a great gym session my dude, when I get home I'll text you my secret protein shake recipe!''
Tom:''Yeah that was intense. Sounds good, ttyl!''