Topics is a word or phrase that helps describe what content is about. Each piece of content should have at least one topic, but try to limit them to no more than six.
Topics are not to be confused with the ubiquitous "Tags" found on many websites. Tags are used to tag content, even if it is not important. Tags can become so numerous, many become meaningless, and since people love to overuse them we often consider them to be nonsense.
Topics are more meaningful because they are fewer in number, straight to the point, and even handle subtopics. When a page has a topic the page is based on that topic, and not some word mentioned in passing which is not very useful.