Tone locking is simply a module used by RepeaterBook.com to prevent the release of the Uplink and Downlink tones associated with a repeater. This is only done at the request of a repeater owner.
It is believed that this is primarily requested as a method to limit access to a repeater and afford privacy on a closed or private repeater.
However, most modern amateur radios include a tone scan function and modern scanners can locate a tone in less than a second. If all else fails, a user could simply try each tone until one successfully activated the repeater. This is a poor attempt at limiting access to a repeater that beacons its existence every time it is used.
The only way to know if tone locking is being used on a RepeaterBook listing, you must view the repeater on its details page. If the uplink/downlink tone field label is missing, then the repeater's tone is being hidden. Otherwise, the field is simply empty. RepeaterBook encourages listings to include the “CSQ” tag, for carrier squelch, on all repeater listings where no uplink tone is required to operate the repeater in order to reduce confusion.