Keywords are words that help search engines know what your article is about. For example, if you write an article about buying guitars, you would want the main keyword to be guitars. So that would be one keyword and then other keywords might end up being bass, acoustic, electric guitars and buying guitars.
The word guitar should appear naturally in your article and around 3% density. So for a 1000 word article, the word guitar could appear 30 times which would be 3%. The other words can appear in the article anywhere between 0.5 and 2% of the time.
Anymore than 3% and Google considers that keyword stuffing. I have always found it best to just write an article naturally and not worry about keywords. I think the biggest worry is having the word guitar too many times instead of too few.
Since in this example, guitar is your main keyword, guitar should also be in the title. It is always best to have at least one keyword in the title.
Keywords just tell the search engines your title is what your article is about. And they should appear naturally throughout the article.
This is a good website to check your keywords density before you put it online. Just copy and paste your article into it and look at all the densities of keywords...