Plain English Meaning
The air is thinner than usual, so the airplane does not perform as well. It needs more runway to take off, climbs more slowly, and lands using more distance.
Full Definition
A condition in which the density altitude is significantly higher than the actual field elevation, meaning the air is thinner than standard. It typically results from high temperature, high elevation, high humidity, or a combination of these. High density altitude reduces engine power, propeller efficiency, and wing lift, which lengthens takeoff and landing distances and decreases .