They will be the time it takes the probe to get the response string back from the device. Have a look at, as an example, the HTTP probe. You will see it sends a string and awaits the receive string. If the receive string is not returned it will assume the service is down. So the time it takes for the device to return the receive string is the time that is graphed. Other probes are just set for Connect Only, thus aslong as the service responds to the probe it will not care what the receive string is and will assume the service is up.
Hope that helps.
Don't forget to give Karma if we help you out