A double locking tremolo is a type of bridge of an electric guitar. Floyd Rose is the organization that licenses, distributes and manufactures the original double locking tremolo, also known as the Floyd Rose Locking Tremolo, invented by Floyd D. Rose. Floyd Rose owns the patents on the design, and licenses these patents to several manufacturers.
A double locking tremolo is actually part of a double locking system that consists of a lock at the nut of the guitar, which prevents the tuning heads from being used and holds the strings taut and a "floating bridge", where the other ends of the strings are also vise-locked, hence, "double-locking".
The locking system helps to keep the strings in tune while the strings are slackened to a degree which was not achieved with older tremolo systems, such as those found on Fender Stratocaster, allowing dive bombs, a rapid lowering of the pitch of a note. Since the tuning heads are ineffectual with the lock in place, the Floyd Rose bridge has heads for fine tuning; the guitar is tuned before the lock is put on, then fine tuned afterwards. Each guitar incorporated with a Floyd Rose tremolo system has springs put in the back of the hardware which create balanced forces with the strings, thus keeping the tremolo in a "floating" state.