As has been covered, it is short for "involuntarily celebate" but means more than just "someone who can't get laid". It's a vile subculture that deeply loathes women, thinks of themselves as ideal men who are victims of women who withhold sex, and are generally just disgusting and pathetic. This BBC article discusses 2 incels/mass murderers and you can read about them in some of their own words. Other famous incels include the dude who advocated for the legalization of rape on private property (though IIRC, they later claimed that was a joke after) and rape is a common subject of their discussions. ("Rapecel" is a charming subset of the incel brotherhood. And mass murder is a fairly common topic of converation among these lads.)
They call attractive women, who they hate the most because they feel these women should be fucking them but aren't as "Stacys". Average women are "Beckys" and men who are sort of the typical "high school quarterback", attract-all-the-women archetype are "Chads". Strangely, the Beckys are often the focus of their rage. They feel as though Stacy will never fuck them, but Becky is pathetic and unattractive enough that she should be grateful for their attention. They sometimes refer to women as "Female Humanoid Organisms", or FHOs. In other words, there is a whole repulsive sub-culture that goes along with being an incel; it's way more than "it's been a few months since I've gotten it in".
