When you see Cluster: FooBar in RecentChanges is tells you that some pages belonging to the FooBar cluster have changed. We call this a cluster link.
Using clusters, people can edit many pages without HijackingRecentChanges. If you are interested in the FooBar subject, click on the link and you will be taken to the FooBar page. The FooBar page is just an ordinary page, but when you visit it using the cluster link, the list of recent changes for all pages in that cluster will be appended to the page.
The RecentChanges output looks something like:
Could we also (or maybe only) have the SubPage be an href?
See also: PageCluster.