A frequency count reduces a body of text to a ranked list of its words and how many times each one occurs. It is the simplest possible summary of what a text is about, and often a surprisingly honest one.
What the ranking reveals
Strip out the structural words and the top twenty remaining terms are, roughly, the subject. If they are not the subject you intended, the text is about something other than what you meant it to be about.
Stop words dominate without filtering
the, and, of and to will always top an unfiltered count. Removing them first is what makes the result meaningful.
Common applications
Profiling a competitor's page, checking a draft against its intended topic, or finding the terminology that recurs across a set of customer reviews.
Try it: Word Frequency Counter on SeoWolf's Notepad.