When creating or editing a wiki page or a forum post, below the edit area you will see a button "Labels" which you can click to add labels to your page.

There are couple of benefits to using labels:

1) It helps Google index your content. Each label you add to your page will be added to the HTML meta data as keyword entries. For example, this page will show:

<META name="KEYWORDS" content="howto, labels, google, index, rank">

So using good, high-quality labels will help your content gain exposure and be discovered by outsiders. You spent time and energy writing that page or forum post, you deserve some Google love as reward!

2) It helps people discover similar content to yours. When using this site's search functionality (the search box on the top right corner of the page), similarly labeled content is offered to the user who created the search query. It helps keep related content together (think tag clouds)