I recently flew on a Boeing 737-800 and found these arrow markings on the wing. What are they for?
They are the overwing exit markings. You can see them in full in the following photo.
This is because as already noted in another answer, B737 does not have self inflating slides for its overwing exits and as such, the passengers need some guidance for exiting, to prevent them from going the wrong way.
The above image shows the procedure for using the overwing exit.
They direct people who've used the over-wing emergency exits to proceed to the back of the wing, where the flaps are (hopefully) extended, giving them something to slide down to get to the ground. The leading edge would be a much harder trip down, AND if the engine is still running, you're about to run forward into the area where you're at risk of being sucked into the running engine. If you went aft and get caught in the jet-blast, you may tumble, but that's better than hitting the spinning fan blades!