Your Time Machine backup disk might not always be available, so Time Machine also stores some of its backups on your Mac. These backups are called local snapshots.
How to use local snapshots
If you enter Time Machine when your backup disk isn't available, Time Machine automatically uses local snapshots to help you restore files. Reconnect your backup disk to make even more backups available.
How local snapshots use storage space
You don't need to think about how much storage space local snapshots are using, because they don't use space needed for tasks like downloading files, copying files, or installing new software.
Your Mac counts the space used by snapshots as available storage. Even so, Time Machine stores snapshots only on disks that have plenty of free space, and it automatically deletes snapshots as they age or as space is needed for other things.
If you want to delete local snapshots manually, just turn off Time Machine and give it a few minutes to automatically delete them. When you turn Time Machine back on, it remembers your previous backup disks.
How often local snapshots are saved
Time Machine saves one snapshot of your startup disk approximately every hour, and keeps it for 24 hours. It keeps an additional snapshot of your last successful Time Machine backup until space is needed. And in High Sierra or later, another snapshot is saved before installing any macOS update.