An aircraft is described as dirty when it has its landing gear down and flaps extended, creating high drag and lower aerodynamic efficiency. Pilots use this configuration to slow the aircraft and steepen the descent angle during an approach to land. The opposite state, with everything retracted and flush, is called clean.
We're coming in hot, so let's get dirty early to bleed off this airspeed.
Rookies sometimes forget that flying dirty drastically increases fuel consumption and reduces glide range, so you never want to configure too early if you are low on fuel.
Related terms
Clean
An aircraft configuration where all retractable gear, flaps, and external stores are stowed or removed to minimize drag.
Flap
A hinged surface on the trailing edge of an aircraft wing used to increase lift and drag.
Approach
The final phase of a flight where an aircraft aligns with the runway for landing.