If you are lower and the air is thicker (more dense) then you have more Oxygen per cubic foot so each gulp of air that the engine takes will contain more oxygen and the engine will perform well. As the air thins out or you get higher then the air is literally thinner with less oxygen per gulp of the engine.
There are many of us here who have fallen into the trap of flying fully loaded when the air is hot (less dense) or the airport is high, only to find that the aircraft struggles to get up (Most of us will have got away with it but learned a valuable lesson from it)