To be used in place of 'well', when one feels there is no more to say. This word was originally brought to the peoples' attention in 1994, from Jim Carrey in the cult classic, Dumb & Dumber.
This is simply one of the many words and phrases that continue to be used in teen conversation today.
Initial use of/birth of the word: "Hey fellas. Big Gulps, huh? Welp, see ya later!"
"'Eyy, Jim! I just finished the job on that engine repair, so, feel free to take the credit for it as usual! Welp, That's my queue to leave, so, I suppose I'll see your rotten self tomorrow! Later!"