Indexed files that have a corrupt index can be recovered using this option. Indices can become corrupt if, for example, a power loss occurs when a program has opened and changed an index file, but has not yet closed the file. A flag, indicating that this has occurred, is detected in the file and reported by subsequent accesses to that file. No further operations can be done on the file until its integrity has been recovered.
Rebuild rebuilds just the index, which is all that is required for the recovery. If the index is already in existence, Rebuild tries to recover the index using the information in that file.