This is an acronym version of the phrase "right now", and it's considered to be very useful when you happen to just want to mention (multiple times) that "something is happening with you in the present time", or "in that very moment in time" (that very time you just texted it).
Friend #1 "Arite dude, r u goin to the movie with me or not?! The movie starts in like 25 minutes. Write me back man!"
Friend #2 "Naw, not 2day; I'm way too busy with my hmwk *rt nw*" ........ "Oh crap, dude I gotta go anyway *rt nw* bcuz my parents are calling me for dinner. Later buddy".
Friend #1 "Oh ok no prob. Yeah ttyl buddy".