A tag is a single keyword or phrase that describes the topic, theme, or subject of a post. You can add as many tags as you want and so can other community members. For example, in a post about a mouse, you might add these tags: mouse, USB mouse, optical mouse, wireless, DPI.
Tagging is a way to help other users discover interesting posts. It's also a way to organize content in the community that you think is related. When you apply tags to a post, you add to the value by providing another way for people to find it.