Plain English Meaning
How much weight the airplane is allowed to carry on top of itself — people, bags, and fuel included. It's what's left over after you subtract the airplane's own weight from the maximum weight it's certified to fly at.
Full Definition
The weight of the pilot, crew, passengers, baggage, usable fuel, and any drainable oil — calculated as the difference between the airplane's maximum allowable gross weight and its empty weight. Useful load represents everything the airplane can carry beyond its own structure and permanently installed equipment.
Context Anchor
You will see useful load in weight-and-balance planning, especially when deciding how much fuel, baggage, and passenger weight the airplane can legally and safely carry.