Stoop means to lean your head and torso forward and down. If you're six feet tall and you tour a historical building, you'll have to stoop to get through the low doorways.
Stoop comes from the same root as steep. You can stoop to clear a doorway, or stoop metaphorically, by lower your morals. If you used to be an honest garbageman but now haul "garbage" for the mafia, you have "stooped to their level." If you slouch and droop, you're stooping. When owls quickly descend on their prey, that's also called stooping. Stoop is also the term for the stairs leading up to a front entrance of a house.