The web proxy cache keeps a cached copy of web pages retrieved by the TMG firewall on behalf of clients on your network. This can really speed up web browsing. The problem is that sometimes that cache can get corrupt – and that can create a variety of issues. Check out this blog post for some insights into the situation
http://snowvm.com/2013/04/03/corrupt-forefront-tmg-disk-cache/
HTH,
Deb