Weight of a Keyword is a ratio of frequency of keyword use to the total amount of words in a page that is indexed in a percentage. In a general case increase of a keyword weight in a page leads to increase of its relevance. But the limit, excess of which is estimated as spam, leads to the fact of the lowering this page position and page position to issue. The search systems recommend to set the weight up to 3 %.
Weight of a keyword or the key phrase also depends on where it's in a page. If the keywords are in h1-h3 or title tags, they have greater weight.