Keyword weight or Keyword Density is a measure of how often a keyword is found in a specific area of the Web page like a title, heading, anchor name, visible text, etc. Unlike keyword frequency, which is just a count, keyword weight is a ratio.
Keyword weight will depend on the type of keyword, that is if the keyword is a single word or phrase. If the keyword includes two or more words, for example, 'XYZ services', every word in the key phrase (i.e. both 'XYZ' and 'services') contributes to the weight ratio in the weight formula, and not as one keyword ('XYZ services').
Keyword weight is calculated as the number of words in the key phrase multiplied by frequency and divided by the total number of words (including the keyword).
Example: The title of a Web page is 'Get Best XYZ Services'. Keyword weight for 'XYZ services' is 2*1/4*100%=50%. If you reduce the number of words in the title by removing the word 'get', so the title becomes 'Best XYZ Services', than the keyword weight will be larger: 2*1/3*100%=67%. Finally, if you only keep 'XYZ Services' in the title, the keyword weight will become 100% -- 2*1/2*100%.
So, to increase the keyword weight, you should either add some more keywords or reduce the number of words in the page area. The proportion of the keywords to all words will become larger, so will the keyword weight.
Many search engines calculate keyword weight when they rank pages for a particular keyword. Normally, high keyword weight tell search engines that the keyword is extremely important in the text; however, a weight that is too high can make search engines suspect you of spamming and they will penalize your Web site's rankings.