OK, what does it actually mean!
When shrinking a database in SQL you are presented with the option “Reorganize files before releasing unused space. Selecting this option may affect performance.”
If ticked it is the equivalent of running DBCC SHRINKDATABASE with a target_percent set, if it is unticked it is the equivalent of DBCC SHRINKDATABASE with the TRUNCATEONLY option.
Ticking the box may affect performance while shrinking the database so it may take longer BUT it will not affect the performance of the database once shrunk.