Handlers tell Apache (the web server software) what to do with certain types of files. For example, .cgi and .pl extensions are processed with Perl. You could make Apache process .scr files with Perl by using the appropriate handlers. To add a handler, simply type in the handler name and extension (without a period) and click on Add.
To access Apache handlers, click on the "Advanced Tools" icon from the main control panel menu, then click the "Apache Handlers" icon.
In the above example, Apache would handle all .ima files as IMAP types, even though the default extension for image maps is .map.
For the Evolution theme the same functionality can be found under the advanced features tab