It is just basically a short term of the word what do you mean.
There's also many other words that we can also use it shortly. These words we can also use them when messaging each other instead of just tying the whole sentence which is very long and not cool looking.
More words:
ROFL means Rolling on floor laughing.
STFU means Shut the *freak* up.
LMK means Let me know.
ILY means I love you.
YOLO means You only live once.
SMH means Shaking my head.
LMFAO means Laughing my freaking *a* off.
NVM means Never mind.