Keyword density refers to how often you write your keywords within a piece of web content. For example, if you wrote a 400 word blog and featured your keyword 4 times, your keyword density would be 1%.
There are a number of resources online that help with determining keyword density, including some overly complicated calculators and formulas, but as the idea behind “content is king” has developed keyword density has become regarded as less important.
If you add a meta keyword tag, and the keyword is then in the title, page URL, and mentioned even once in the content, that is often enough.
Don’t dilute your content and ruin its readability by putting additional keywords in just for the sake of it. If you write your web content in a natural manner, keywords will find their way in without them feeling forced or contrived.