What the “TRIM” is? What is the relationship between trim and garbage collection?

TRIM is an OS (Operating System) command set, which was started supporting in Windows 7. When user deletes a file, OS sends TRIM to inform SSD about the operation, then SSD marks the specific storage units to invalid state. Once the GC operates, GC could be more precise and efficient to erase the storage unit without unnecessary reallocation of data in flash memory.