"...we tend to explain things in terms of things that we understand."
What Matt is describing in that almost throwaway line is the progressive method of explanation. When faced with something complex, which as a whole we might not understand, we tend to break it down into its component parts and seek to understand them. If these components parts are also complex, we will further break them down until we get to a level we do understand. This is a lot like maths. Higher mathematics can be complicated, but it's built up from lesser principles, all of which can be ultimately reduced to something basic and comprehensible.
Likewise engineering, technology, and indeed all of education can be seen as a series of hierarchical steps based on something lower down the tree of complexity. We explain things in terms of other things for which we already have explanations.
Intelligent design proponents don't use this method, so their claim of "explanatory power" is bogus. Trying to explain something in terms of something else that we don't understand is clearly a non-starter. It's not helpful, and it's not explanatory.