The promote tool makes it eligible let your wiki appear on the home page of Wikia: http://www.wikia.com/.
You can do this on the "Special:Promote" on your wiki. You can go there via the Admin Dashboard or just by typing it in the url of your wiki.
First you add a headline to let people read the full text you're going to write about your wiki. In this text you tell something of your wiki, the topic etc. At last you can add some pictures of your topic. Don't forget to publish it.