The pagestats directive is supplied by the pagestats plugin.

This directive can generate stats about how pages link to each other. It can produce either a tag cloud, or a table counting the number of links to each page.

Here's how to use it to create a tag cloud:

[[!pagestats  pages="tags/*"]]

And here's how to create a table of all the pages on the wiki:

[[!pagestats  style="table"]]