DTH is “Device type Handler.“ One of smartthings’ many names for a Device driver.
They are also called “device handlers“, “Device types”, and a couple of other things.
The following community FAQ explains the terminology and the concepts. It was written for the 2015 version of the smartthings app so some of the publishing steps are out of date, but it’s a good basic read anyway as far as understanding the approach for adding custom code to the groovy cloud. (The topic title is a clickable link.)
And for giving you some of the terminology that will still be used with the new platform.