It's measuring the time it takes for you to respond to a request.
Your browser sends the request, it hits one of my servers and is immediately returned back to you. I then cut that time in half to represent half of the round trip... and call it Response Time. Generally it should be close to what you can expect to see when you run a ping but don't confuse it with ping, I've found that there are many more things that can influence the Response Time Test.
I'm still trying to make sense of it myself... it will without a doubt need much more development. I've been reluctant to release it because I don't fully understand everything but I figure the best way to understand it is to get feedback from my users.
Sorry to my Beta Testers for not releasing this to you first... but there really aren't that many active beta testers and I need a wide range of input from many people.
Please help and provide feedback, good or bad.