We use keywords when talking about search engine rankings. The visitors you are targetting will use keywords when searching and you choose the keywords you would like your website to be listed for. Once you have chosen your keywords, this becomes an important part of writing your copy for the site.
Traditionally we have been taught at school and in various writing classes not to repeat ourselves too much. This often means trying to use a number of words which basically all mean the same thing, so as not to seem repetitive.
When you write copy for search engine optimisation you forget all of this and ensure that you write your copy using the keywords you have chosen, regularly and consistently. Obviously there is little point in repeating yourself too much making the content unreadable to the visitor, but you should try and mention the keyword enough so the search engine will be able to know what your page is about and that this topic is the chosen keyword!