What it means
A local snapshot is a recent Time Machine copy held on the Mac's own APFS disk. It can offer older file versions while the usual backup drive is disconnected. Apple says snapshots are generally created around hourly, are kept for a limited time and may be removed when space is needed.
A real-world example
You overwrite a document during travel, with the backup drive at home. Time Machine may show a local version from earlier that day. You can restore it without waiting to reconnect the drive. If the laptop is lost or its internal storage fails, that same-disk snapshot disappears with it.
What to do
Open Time Machine from the folder containing the changed file and look for an earlier version. Reconnect the external destination when possible and verify it has a more durable copy. Do not count a local snapshot as your only backup in a recovery plan.
The distinction that matters
A local snapshot is not the same as an independent backup. Apple's explanation distinguishes on-device snapshots from backups saved to a separate disk and says snapshot dates may not match the latest-backup date shown in settings.
Apple's local snapshot explanation
