The changes command is informational, it doesn't do anything to a checked-out project, but it tells you something about it.

This is simply a quick way to get a list of the files which are different in the source tree (the checkout) and the repository.

There is a bit more information (was a file edited, added or removed?, for instance).

The same information will be displayed if you  fossil status , except there will be some additional repository information displayed first.

See also: fossil status, Reference