To disappoint someone by failing to meet their expectations or fulfill a promise. He let down his team by not showing up for the final match.
To lower something to a lower position. She let down the rope so that he could climb up.
To release or allow something to descend. The mechanic let down the car from the lift after the repairs were completed.
To allow air to escape from something, such as a tire. He accidentally let down the tire while trying to check the pressure.