What does 'A' in the Aptitude package list mean?

The first character of each line indicates the current state of the package. i means the package is installed, and p means that no trace of the package exists on the system. The second character indicates the stored action (if any, otherwise a blank space is displayed) to be performed on the package. If the third character is A, the package was automatically installed.

From the results of man aptitude:

search
           Searches for packages matching one of the patterns supplied on the
           command line. All packages which match any of the given patterns
           will be displayed; for instance, "aptitude search '~N' edit" will
           list all "new" packages and all packages whose name contains
           "edit". For more information on search patterns, see the section
           "Search Patterns" in the aptitude reference manual.

           Unless you pass the -F option, the output of aptitude search will  
           look something like this:  

               i   apt                - Advanced front-end for dpkg
               pi  apt-build          - frontend to apt to build and install architecture optimized packages
               cp  apt-file           - APT package searching utility -- command-
               ihA raptor-utils       - Raptor RDF Parser utilities

           Each search result is listed on a separate line. The first
           character of each line indicates the current state of the package:
           the most common states are p, meaning that no trace of the package
           exists on the system, c, meaning that the package was deleted but
           its configuration files remain on the system, i, meaning that the
           package is installed, and v, meaning that the package is virtual.
           The second character indicates the stored action (if any; otherwise
           a blank space is displayed) to be performed on the package, with
           the most common actions being i, meaning that the package will be
           installed, d, meaning that the package will be deleted, and p,
           meaning that the package and its configuration files will be
           removed. If the third character is A, the package was automatically
           installed.

           For a complete list of the possible state and action flags, see the
           section "Accessing Package Information" in the aptitude reference
           guide. To customize the output of search, see the command-line
           options -F and --sort.

See man aptitude or the embedded help in Aptitude:

If the third character is A, the package was automatically installed.

It means the package has been installed to satisfy dependencies of other packages and it would be removed if the dependent package(s) were removed.

To check which package depends on an automatically installed package, use the command aptitude why <package> or press I in the Aptitude TUI (repeatedly).