This option tells the proxy that WebMask is requesting pages from to not uses its cached copy, but retrieve a fresh copy from the actual site.
WebMask adds "Pragma: no-cache" and "Cache-control: max-age=0" fields to the request headers - the proxy doesn't, however, necessarily obey them.
When refreshing in a browser, these fields are sometimes sent anyway to make sure you get a fresh copy. This option lets you guarantee that this happens. You probably wouldn't want to leave it on permanently, unless you needed to use a caching proxy for some other reason, but you didn't want to use its cache.