Open the Terminal in the Utilities folder and enter or paste the appropriate command line. Press RETURN and enter your admin password when prompted. It will not be echoed.
To turn them ON: sudo tmutil enablelocal
To turn them OFF: sudo tmutil disablelocal
Note that turning them OFF will also delete all existing snapshots.
Don't worry about them. They'll be "thinned" down to one per day for a week, then deleted.
They'll also be deleted if your disk gets over 80% full.
They give you access to things you've changed or deleted in the last week, while you're away from your normal backups on the external HD.
See Time Machine - Frequently Asked Question #30 for the gory details.