They're in a super human society where 80% of the population has special powers called quirks. Katsuki's Quirk allows him to secrete nitroglycerin-like sweat from the palms of his hands and ignite it on command, allowing him to create strong explosions.[2] The more Katsuki sweats, the stronger his explosions become.[12] If Katsuki overuses this power, his forearms will start to ache.