In the past, some webmasters wrote lots of words which are not related with their websites or they wrote a word again and again in order to get more visitors. They though that the more word they wrote, the more visitors visit their websites. They could drive more visitors in the past but now, search engines have been developed and they became more clever. Today, writing a word again and again can cause ban from search engines.
On the other hand, we should write a word more frequently than the others in order to optimize our web pages. For example if we prepare a webpage about "cars" it is ordinary that "car" word we be written more frequently than the others. Thus, search engines think that webpage, which we created, is related with "cars"
In order to determine best keyword density, you can use Keyword Density Tools. You can optimize your websites by fine tuning keyword densities. Keywords should not have density higher than %8-9 and try to arrange their density between %2 and %3