Benefit
You are proficient with your unarmed strike, which deals lethal or nonlethal damage (at your option) equal to 1d4 + your Strength Modifier (if Medium size). Your unarmed attacks count as armed, which means that opponents do not get attacks of opportunity when you attack them unarmed and you may make attacks of opportunity even while unarmed.
Special
A monk gains Improved Unarmed Strike as a bonus feat at 1st level. She need not select it. In addition, a monk deals higher damage with her unarmed attacks than is described here.
Normal
Without this feat, you deal only 1d3 points of nonlethal damage (if Medium size). Unarmed strikes normally provoke attacks of opportunity, and unarmed combatants normally cannot make attacks of opportunity.