I'm not entirely sure what they're supposed to be either. Apparently it's supposed to be a combination of the words "involuntary" and "celibate". But that doesn't entirely work because it's possible to be involuntarily celibate and be fine with it. I think when people talk about incels they're mostly talking about the ones who are involuntarily celibate and pissed off about it. IOW, beating themselves against reality until they get so frustrated they become irrational and annoying.