Keyword density is the number of times your keyword or keyword phrase appears inside content on a web page, compared to the total amount of content. This is considered by SEOs to be part of the on-page SEO for targeting a specific keyword for a site.
So how important is it, really? As the search engines progress and develop further, their algorithms change to fit one desired outcome: judging web sites and pages in the same way humans do. In this way, the highest quality web pages should show up at the top of the search engine results.
Considering this, making sure you have a keyword density of 3-5% is not something that most humans take into account when they visit a page. I don’t know about you, but I personally don’t do a search for a keyword, land on the page, and then count the number of times that keyword appears, do the math, and then dismiss the page if that percentage is outside the magic range. I read the page, see if the content is good and high quality, and make my judgements based on that. And if it’s really good, I might even link to it from one of my own pages.
Too many SEOs worry about fitting keywords X number of times into content, to a degree that often makes the content feel more unnatural. This only hurts their connection with the human visitors to their site.
As it is now, the search engines will look for keywords in content, but having a specific density percentage is not in the top of their search-quality priority list. If the page is specifically on a certain topic, the keywords will naturally appear within the content. Forcing it will generally not help for the search engines, and certainly will not help with your human visitors.
My recommendation is to simply write the content to stay on topic and don’t worry about embedding keywords. If you keep your keywords in the proper core SEO locations, the search engines will know very well what your page is about without having to do any sort of deep content keyword analyzation.