If there is a keyword phrase you are trying to target, and you are writing about it on one of your pages, it would essentially be the distance between all the words in the phrase as they appear on you page. It's a rather silly metric imo, and not very practical to try and measure/evaluate. Basically if you were trying to optimize towards the phrase "online degree program" and you had a sentence like "study online and earn your degree from a variety of programs available at...", that would have a lower keyword proximity vs. saying something like "choose from one of our 50 online degree programs and earn your degree today!".