The program will make sure that no instance of digiKam is running, because it is more safe to have no database access during the optimization process. It then will read the digiKam configuration file and determine the database location. In a final step the database will be optimized by invoking the sqlite command 'VACUUM;' on it. If more then one database is found in this location, cleanup_digikamdb will optimize every database found in this path and below.
For further explanation, see the following description of the VACUUM command from the sqlite3 website:
When an object (table, index, or trigger) is dropped from the database, it leaves behind empty space. This empty space will be reused the next time new information is added to the database. But in the meantime, the database file might be larger than strictly necessary. Also, frequent inserts, updates, and deletes can cause the information in the database to become fragmented - scrattered out all across the database file rather than clustered together in one place.
The VACUUM command cleans the main database by copying its contents to a temporary database file and reloading the original database file from the copy. This eliminates free pages, aligns table data to be contiguous, and otherwise cleans up the database file structure.