Rule of thumb, it means "I agree" or the exclamation "Agreed"

See more here:
http://english.stackexchange.com/questions/274600/meaning-of-word-up-exclamation
http://english.stackexchange.com/questions/218351/what-does-the-expression-word-mean/218356#218356
http://english.stackexchange.com/questions/25086/what-are-the-meaning-and-possible-origin-of-word-and-word-up