A large amount of angry, angry disillusioned people. I'm culturally close enough to understand a bit, if not 100% of it. There's been a lot of cultural change really fast in the West, and increasingly bad economic conditions for the poor, rural and/or uneducated at the same time. As a result, a bubble of people who are completely reactionary and want to tear down the establishment has formed. Trump just managed to mobilize them.
The part I don't really get is the appeal of the guy himself. It's like they want to inflict him on the people they're angry at, as if he's a weapon and not a leader who will be in charge of them.