Wear-Leveling -- Wear Leveling is a process we describe briefly in the definition of Overprovisioning, and the two are very closely tied (as is Garbage Collection). Wear-Leveling is a necessary process unique to SSDs and Flash NAND; the process ensures equal wear across all NAND cells on the disk, due to the limited nature of Flash.
Flash NAND cells have a limited count of Program/Erase Cycles (or P/E Cycles) that, when exhausted, result in a dead drive that will likely enter a limited-time read-only state of write-lock. In short, this limited count of P/E cycles is due to the electrical charge growing weaker with each write/erase cycle performed on the disk, ultimately resulting in less accurate read/writes.
If a single NAND die or cell exhausts its program/erase cycles, the entire drive will enter a locked state (there are exceptions to this in instances where overprovisioned space can be instated to replace the dead unit). To ensure NAND is equally worn, the controller will perform the “wear-leveling” task of relocating (erasing and rewriting elsewhere) data on the disk. This benefits endurance in the long-run.