Wikitext is the markup language used to format content on wikis. It can be used to add photos, tables, bold styles, links, and other visual changes.
It's not necessary to know wikitext in order to edit at Wikia. You can use the Visual mode in the editor to avoid it. However, users who learn it find that they have more control over their editing experience. It can be used in "source" mode in the editor.
Here's a cheat sheet for basic wikitext that you can use while editing.