I am assuming that you are talking about the small 2-2.5mm allen head bolt that is threaded through the pivot knuckle on the lever blade. If that's the case, that is a reach adjustment. Turn it in and it moves the lever blade away from the bar, turn out and the blade moves closer to the bar.
Ah, same thing! Reach adjustment screw. I forgot Shimano puts the adjustment screw on the front of the brake lever housing above and behind the barrel adjuster. Does the same as the 2.5mm bolt on the lever blade, just a different location.