registering your css file like this will suck that file up into the sites main css file and compress it.
This has many advantages for page loading speed.
#1: the file is compressed to as small as it can be
#2: when you work on it your working on an un-compressed copy so its readable.
#3: since the CSS file remains the same across the site, it can be cached by your browser so it does not need to be re-loaded. That equals faster page load times.
/xxModuleName/js/( the file name set in jrCore_register_module_feature() )