What is a Keyword?
A keyword is a word that is going to be placed in your article several times, not just once as that would make every word a keyword. When a spider sees that you have a word placed several times in an article, it will determine that your page may be useful to users that search for such a keyword.
Over Optimizing Your Articles
It is important to note that there is also such a thing as over optimizing your articles for particular words, this is known as keyword stuffing. When you stuff keywords in an article a spider will detect that you are trying to trick it into placing your article high in the search engine results for that word, and will instead penalize your site and your page for doing such. This may even affect the rankings of your other pages or get your site blacklisted from a particular search engine if you are found keyword stuffing too many times.
One of the tools which can be used to save your time for submitting article is automatic article submitter. This tool allows you to put the content keyword too.