Your Time Machine backup disk might not always be available, so Time Machine also stores some of its backups on your built-in startup drive and other local drives. These backups are called local snapshots. One snapshot is saved every hour. If you're using macOS High Sierra, another snapshot is saved before installing any macOS update.
How to use local snapshots
If you enter Time Machine when your backup disk isn't available, all the backups that you can access are from local snapshots. You can restore from these backups or reconnect your backup disk to make other backups available.
How local snapshots use storage space
In macOS High Sierra, Time Machine stores snapshots on every APFS-formatted, all-flash storage device in your Mac or directly connected to your Mac. In earlier macOS versions, Time Machine stores snapshots on the internal startup disk of your Mac notebook computer and any Mac OS Extended (HFS Plus)-formatted storage devices directly connected to that computer.
Snapshots older than 24 hours are automatically deleted. And to make sure that you have storage space when you need it, snapshots are stored only on disks that have plenty of free space. When storage space gets low, additional snapshots are deleted, starting with the oldest. That's why Finder and Get Info windows don't include local snapshots in their calculations of the storage space available on a disk.