This is a greasemonkey userscript that runs on browsers that support userscripts. It does not run on Chrome natively, as Chrome userscripts don't support @require or cross-domain xhr. (See the chrome-extension branch for an extension that does work on Chrome.)

It requests a canonical source for avatar replacements for people on a specific phpBB forum.

It has a hardcoded list of servers to try in order for images and a hardcoded phpBB forum to replace images on. It is intended to be used with the game "Skyrates", at forum.skyrates.net.

NOTE: If you click the link directly from the github page, that normally goes to a page that gives you more information about the file. But since it ends in user.js, your browser tries to install it as a script - so you'll install an HTML file as the script.

Workaround 1: Disable greasemonkey, click the skycust.user.js link, enable greasemonkey, click "raw".
Workaround 2: Use http://github.com/SohumB/skycust/raw/master/skycust.user.js instead.

You really shouldn't be installing it directly from here anyway :P