Tags let you label content using terminology that is familiar to you. For example, a ‘key’ to you might be very different than the ‘key’ label used by someone else. With tags, you can choose the label that you want to use, and not be forced to use someone else's label.
Once you start adding tags, you can use them to help you find items on 3D ContentCentral, or think of them as bookmarks to help you recall items in the future. Tags also let you see how other users label items, and that can help you discover other items with similar qualities. You can add your own tags to any content, or use someone's tag that is already there.
For more information about tags, see the Wikipedia article.