Anything that you see int he parks, from lights, to street signs to doors to rides to ride systems to vehicles to the castles and Tree of life.. they DO EVERYTHING!
If it's anything like engineering, which I assume it is, they do whatever it takes to turn an idea into reality. This includes not only designing and building custom hardware and software, but also helping to decide which components to buy and which to make; selection of purchased components; and integration of the various components (IE figuring out how to get all the various components, some of which are custom built and others of which are bought, probably from multiple vendors, to all work together).
My understanding has always been that imagineering is what I've just described, with a heavy dose of creativity on top. I believe the creative aspect is what separates engineers like me from Imagineers.
They don't actually MAKE the stuff usually (they hire outsiders for that) but they make the ideas and (depending on your field) they figure out how to make those ideas a reality.