Overprovisioning (OP) -- Overprovisioned space is present in all solid-state drives. Overprovisioned space is inaccessible to the user and cannot store data, but still resides in the dies on the SSD. This space is used as a reserve for background tasks like wear-leveling, garbage collection, and as a back-up in the event of catastrophic cell (SSD) failure. For most consumer drives, the user can expect 7% overprovisioned space (if you see a 120GB SSD, it likely has 128GB of capacity with 8GB reserved for OP).
On some Flash Controllers, overprovisioned space speeds up active read/write processes by assisting with data compression (compressing the data to a smaller, more easily written state).
SSDs do not and cannot store data in the same fashion as a hard drive, where data effectively sits in a single spot (magnetically written) until otherwise relocated by a defrag. SSDs must actively erase and rewrite all data stored on the disk, a process we describe here and in more depth here. This process exhausts the limited Program/Erase cycles on the disk, but is necessary to ensure equal “Wear-Leveling” across all the NAND. The SSD must erase and relocate blocks of data at a time, otherwise some cells would die (exhaust their P/E cycles) before others, which would cause failure of the entire disk.
Overprovisioned space is used for swapping data during this process of wear-leveling.