Those letters beside files in the Project Navigator of Xcode show the status of files that are under version control systems, such as SVN or Git. So, for instance:
- M – means the file has changed and it should be merged into SCM
- A – means this is a new file and should be added to SCM
- U – means this is a newer version of a file on SCM and you need to update it
- ? – means the file has not been added to source control
- etc…
P.S. You can find list of statuses (at least for SVN) here