A lock lever is each of the metal plates inside a lever lock. Each one has a gate and a spring, and the teeth of the key lift them to exactly the right height so the bolt can pass. If a single lever sits too high or too low, the lock will not open.
The more levers a lock has, the more key combinations it allows. Even so, these locks can be manipulated through the keyhole, and if a key is lost the whole lock has to be replaced or re-levered, which is more work than changing a cylinder.
If your door has a lever lock and you have lost a key, consider converting it to a cylinder: next time, changing the cylinder will be enough. We explain it under lever to cylinder conversion and lock installation.