Plain English Meaning
Stopping a descent at the altitude you want and settling into steady, level flight there, with the right power set and the airspeed you want.
Full Definition
The maneuver of transitioning an airplane from a descending flightpath to level flight at a chosen altitude, accomplished by smoothly increasing pitch attitude and adjusting power to arrive at the target altitude with the desired airspeed and a stable, level attitude.
Context Anchor
You encounter this during descents, approach planning, traffic pattern work, and any time you are assigned or choose an altitude after descending.