Keyword weight is the percentage or concentration of keywords on your page in relation to all other words on the page. A "keyword" can be either a single word, or a short phrase.
Keyword weight refers to the number of keywords appearing in the page area divided by the total number of words appearing in that area. More accurately, the formula WebPosition computes the weight a little differently depending on whether the keyword is a single word or a multi-word phrase.
Weight formula:
(number of words in the keyword phrase * frequency) / total words in area
Therefore, your weight will logically increase when the number of keywords on the page increases or the number of words on the page decreases.
Some search engines consider keyword weight when determining the rank of your page for a particular keyword search. In general, the higher the weight the better, but only to a point. If your weight becomes too high, you may be penalized.
WARNING: Simply dividing the frequency by the total words on the Page Analysis table will not yield the correct weight when the keyword is a multi-word phrase.
For example, if the area had only three words in it:
My Blue Widgets
And the keyword phrase was "Blue Widgets," then the following statistics would be displayed:
Frequency: 1
Total Words: 3
Weight: 66%
The reason weight is not 1 divided by 3 in this case, or 33.3%, is that the keyword occupies two of the three "word slots" in the title, commanding a 66% weight. If we didn't compute it this way when doing an exact search, then a title of:
Blue Widgets
would yield a frequency of one, and a word count of two. However, it's obvious in this example that simply dividing 1 frequency by 2 words is not correct since it would yield a 50% weight rather than a 100%. The weight must logically be 100% because there's no way a title called Blue Widgets with a keyword phrase of Blue Widgets could have a higher weight. 100% of the words in the area are already keywords, thus yielding a 100% weight.
Ultimately, it doesn't matter much how weight is calculated exactly so long as it is consistently applied for each page analyzed. If top ranking pages tend to have a 3% weight as computed by the Page Critic, then 3% is what you want to shoot for regardless of how the 3% if computed. The goal is to emulate the statistics of top ranking pages. If you focus on emulating the frequency and word count of top ranking pages, your weight will generally fall in line.