Unlike hard disk drives (HDDs), NAND flash memory cannot overwrite existing data they must first erase old data before writing new data to the same location. With SSDs, GC is the name for the process of relocating existing data to new locations and allowing the surrounding invalid data to be erased. Flash memory is divided into blocks, which is further divided in pages. Data can be written directly into an empty page, but only whole blocks can be erased. Therefore, to reclaim the space taken up by invalid data, all the valid data from one block must be first copied and written into the empty pages of a new block. Only then can the invalid data in the original block be erased, making it ready for new valid data to be written.

http://www.thessdreview.com/daily-news/latest-buzz/garbage-collection-and-trim-in-ssds-explained-an-ssd-primer/