Tags are essentially keywords. When you're about to post an item, like a video, photo, blog post or document, select or type in the keywords that come to mind in the Tags field. This will increase the search relevance of the item and help others find it in the future.
Photo credit: familymwr
On many pages in the system you'll find a Tag Cloud that shows the most commonly used tags in that part of the community. You can click on any of the tags to find items that have been tagged with that word.