Plain English Meaning
The pushback the air gives the aircraft as it moves forward. The faster you go or the more lift the wings make, the more the air pushes back against you.
Full Definition
The aerodynamic force that acts on an aircraft parallel to and opposite the direction of its flight path through the air, resisting forward motion. Drag is produced by air friction along the aircraft's surfaces and by the production of lift, and it must be overcome by thrust for the aircraft to maintain or increase speed.
Context Anchor
You will see drag discussed in aircraft performance, climbs, glides, approaches, and any situation where airspeed or power changes matter.