Tags in WordPress is a way to “group” similar posts which may or may not belong to the same WordPress category. What tags are used for is to find similar content regardless of posts category and a more focused way of listing same-topic content of a blog.
These are usually keywords defined at the time of making a post in the Tags Section.
Although optional but adding tags to WordPress post is strongly recommended as WordPress tags and SEO goes hand in hand.
Keep your tags related to your content. Ideally, use one word only and not more than three words maximumWoopoo's Tip
Unlike posts, you can’t add tags to WordPress page and there is a reason for it. Pages in WordPress are mostly used for static information like blog’s About Page, Contact Page and such pages don’t need to be listed. Instead pages always find their home usually in menus or footer areas.
Don’t confuse WordPress Post tags with HTML tags or hash tags which are totally different terms.Woopoo's Caution