if there are multiple users, other user can Not access the database during shrink (offline shrink).
The database is backed up first, then is shrunk.
When some data are deleted from the database, the hard disk space are not released immediately. The space are reserved for new inserted data. The approach can improve database performance.
Only shrink the database after a large amount of data are deleted or the hard disk space are really want to be released for other purpose.
If the database is large, it takes some time to shrink. If there the operation fails, e.g. power off during shrink, the backup file can be used to restore the database.