Wear leveling is a technique that extends the lifespan of an SSD by distributing write operations equally across all blocks of NAND.
This is important because all NAND flash memory produced today have a limited number of erase/write cycles. Once the endurance limit has been reached, the expired blocks would no longer be able to be rewritten with new data.
To preserve the limited number of erase/write cycles, the SSD controller doesn’t actually perform an erase operation on a data block when it’s issued a deletion command. Instead, it simply marks the block as invalid, which simply means that the data can be safely overwritten.
When a new write command is issued, the controller will route that write command to blocks of NAND with the least amount of write/erase cycles. This ensures that all NAND on the entire drive is being degraded at the same rate, allowing the drive to continue to operate until its nearing its end of life.