To "Tag" your article means to add keywords to it.
It is kind of like playing a game of Tag because you are tagging your article "it" by adding specific keywords that define your article.
So overall to tag your article means to add keywords to your article.
For blogger the label box is the tag box.
Just click on the above pictures to enlarge them.
What are Keywords?
One to two words describing your article or what your article is about.
How can you utilize Keywords, besides adding it to the Tags: box?
When you write an article first decide what you will write about. After you know the subject and the overall idea for your article come up with some keywords (or Key-Words : meaning important words) and make a list.
While you are writing your article use those words as often as you can without taking away from the content of your article. The more keywords your article repeats the easier it is for search engine users to find. (ie Google, Bing, Yahoo)
How many keywords or tags should I use?
You should only use 5-8 tags when you are adding your tags to the tag box at the end of your article, however you can repeat your keywords in your article as many times as is necessary without making it look obvious that you are repeating them.
How much success does using Keywords or Tags guarantee me?
There are no guarantee's but writing in correct form using correct tags definitely doesn't hurt your chances for success. There are keywords I have tagged articles with that have gotten me to the #1 ranking spot on Google when readers search for that keyword or something very similar.
There are other keywords that don't seem to bring my ranking up. It's really trial and error. Just keep trying even when it seems like you are failing as long as you are continuing you will succeed.