To repair a database, right-click on the database name and select "Repair" from the context menu. The selected database is checked for possible errors and inconsistent data storage and gets fixed if necessary. Furthermore, all indexes within the database are rebuilded.
This command creates a global write lock and blocks other operations until it is finished.
This operation requires additional disk space equal to the size of your current documents plus 2 gigabytes.
More detailed explanations on this functionality can be found on http://docs.mongodb.org/manual/reference/command/repairDatabase/.