Older file systems required occasional cleanup. You’d run a disk defragmentation program and wait for it to finish before you could do anything else.
Modern file systems do this automatically in the background during normal operation. This means, of course, that the hard drive’s LED often lights up even when you’re not actively using it. (This used to be one of the indicators of a virus infection 
)
That’s why btrfs doesn’t need defragmentation (with very rare exceptions). btrfs simply tries to keep its disk space clean continuously (this isn’t unnecessary work and stops when everything is clean).
This will not harm an SSD !
If you’re coming from Windows, there’s a lot to learn. Even if you’re switching from ext4 to btrfs, you should do some research.
use noatime with btrfs, manual rollback, rescue data, unable to boot → out of space