Basically, they count the number of hits returned by a search for "{language} programming" across a wide variety of search engines and websites, and rank the languages according to which one yields the most hits.
It is an interesting metric, but I just can't bring myself to see it as the measure of popularity that it is presented as. The number of page hits for a language is a function of many things. Popularity is one. Others include age, complexity, and how confusing/difficult-to-learn the language or its libraries are. I'm guessing there's a marketability angle, too - for economic reasons, there may not be as many codepoints being poured onto languages whose users are less likely to purchase commercial tools or consulting services.
I can think of no better illustration of this than the 5th, 6th and 7th places in this month's rankings. Does anyone really believe that JavaScript is only the 7th most popular programming language these days? Does anyone really believe that both C# and Visual Basic are more popular than JavaScript? If you take the TIOBE score as just a measure of popularity, it would seem to suggest that .NET developers are collectively over 3 times as numerous as JavaScript developers.