Tags (also known as keywords) are a convenient way to categorise and group your blog entries. If you tag all your entries about your pet cat with "cat" for example, then later you'll be able to view all those entries together with a single click. Even better, other people will be able to view just the entries about your cat, if that's what they find particularly interesting on your blog. And for people who really like cats a lot, it's also possible to see all the entries on every blog which have been tagged with "cat". So it's worth taking a moment to think about some useful tags for your entry so that you can sort and review your entries later and so that your entries are categorised together with other peoples' entries on the same topic.
In particular, it's worth trying to make sure that you use the same tag that other people have already used when writing about the same subject. For example, if twenty other people have tagged their entries "cat" but you choose to tag your entry "moggy", then people who read or bookmark the "cat" tag won't see your entry. The more people use a common set of tags, the more useful and powerful tags become.
Note that tags can't have spaces in them. If you wanted a tag "History of Art", you would use "history_of_art" as the tag which would then be displayed on your entry as "History Of Art".