Definition
Gradually increasing the demand on the body over time, through more weight, reps, sets or better quality, so it keeps adapting.
It is the principle every other programming term serves. The body adapts to what it is asked to do, so the ask has to keep rising.
The levers, roughly in the order a beginner should pull them: more weight, more reps at the same weight, more sets, a fuller range of motion, better control of the tempo, less rest, and more sessions per week. Beginners can progress weekly, intermediates every two to four weeks, advanced lifters block by block.
The usual failure is chasing the weight alone until form gives way. Overload should be visible in the training log, not felt in the joints.