Plain English Meaning
It's the drag the airplane creates simply by pushing through the air. The faster you fly, the more of it there is. It has nothing to do with making lift -- it's just the air resisting the airplane's shape, surfaces, and joints.
Full Definition
The drag produced by an aircraft moving through the air that is not associated with the production of lift. It includes form drag (caused by the shape of the aircraft pushing air aside), skin friction drag (caused by air rubbing along the aircraft's surfaces), and interference drag (caused by air currents from different parts of the aircraft mixing together). Parasite drag increases as airspeed increases, roughly with the square of the speed.
Context Anchor
Seen in takeoff and ground effect discussions, especially when comparing drag that decreases near the ground with drag that still increases as the airplane speeds up.