In a nutshell, they're hashtags. They label your posts with quick topics and keywords to help people immediately know the theme or primary subject of your post.
They also serve as navigation buttons linking you directly with all posts in the forum that are tagged with the same topic.
To add tags to your post, type them in the box indicated in the screen shot below, then press ENTER to signal the tag is finished. You can enter more than one tag and each has a character limit of 20.