Keyword density and placement are important parts of optimizing your articles for search engines. Search engine spiders scan a page in a way that makes it important to place your keywords where they will be detected and recognized as a keyword, so that your article will come up when someone searches for that keyword.
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.
What is Keyword Density?
Keyword density is how many times your keyword is placed in your article. Most use a percentage to determine how many times they will put a keyword in an article. For instance, if you have a 500 word article and want to achieve a keyword density of 5%, then you will need to have the keyword in your article exactly 25 times. You can find hundreds of resources and guides recommending one keyword density over another and the reasons behind the logic, however, in the end you will have to determine which density is more profitable for your articles. Each webmaster as their own density that they like to achieve based on past results. As long as you dont over optimize and you are making sufficient profit from your rankings, then you can choose whatever keyword density you like.
The Right Density
No matter what exact density you choose, it is important to place keywords so that there are more at the beginning and end to produce an hour glass effect. Having the right keyword density in your article makes it more likely that you will make money off of that article because it will rise in the search engine results and be seen by more people.