Keyword density is a measure of how many times your targeted keyword or keyphrase is being used in the content of your web page.
By repeating your important keyword, your increase the prominence of those words to the search engines. This will help the search engines to identify your target keyword and rank the web page accordingly.
However, there is a penalty if you keyword stuff or use the same keyword too many times in an unnatural way.

Repeating identical keywords or phrases for more than 3 times in the body of the the page may actually damage than good as far as the search engine algorithm is concerned.
Instead, use words with similar meaning - just like when you write naturally. Search engines are smart enough to recognise words with similar meaning and variations such as man, men, male, boy, boys, masculine and etc.
In each page, focus on one or two keywords but not more than three keywords so that it is apparent to the search engine what your page is about.
Keyword Positioning
As much as possible, use the keyword or keyphrase in the heading of the page, in the first sentence of the first paragraph and near the bottom of the page.
As a rule of thumb, your most important keywords - the exact phrase should not be repeated for more than 2% of the total word count. If you have a 500 words page, keep the keywords count to less than 10 times.