Xcode letters beside files in Project Navigator

A - Added (This is a new file that has been added to the repository)

C - Conflict (There is a conflict in the file)

D - Deleted (a file has been deleted)

M - Modified (An existing file has been changed)

R - Renamed (The file has been renamed)

U - Untracked (The file is new or has been changed but has not been added to the repository yet)

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