“Incel” is short for “involuntarily celibate,” meaning they are men who have centered their identity on wanting to have sex with women, but with no women “giving” them sex. Incels objectify women and sex, claiming they have a right to have women want to have sex with them. Incels believe they are being unfairly denied this sex because of the few sexually attractive men (”Chads”), and because feminism told women they could refuse to have sex. Some incels believe their biology (e.g., skull shape) means no women will “give” them sex. They will be forever alone, without sex, and unhappy. The incel community has produced multiple mass murderers and terrorist attacks.
Often, this is a self reinforcing cycle. Dwelling on the fact that women aren't interested in you won't make you more appealing. Additionally with the rise in standards from social media, incels also won't go for women similar to them. Couple these factors together produces no good.