Hard one to explain to somebody who hasn't experienced it. How old are you and in what context are you wondering about this?
it's when the sexual region of the male and female have muscle reflexes due to sexual activities, and they feel really really good.
Orgasm is the pinnacle of female sexual gratifaction, the highest pleasure a women can have. When a women has the ultimate "genuine" orgasm, lots of things happen, some examples are profuse sweating(like she's been in a sauna for an hour) sopping wet all over, drenched hair, engorged breasts, dialted pupils, very fast heart rate 200+ beats per minute, the base of the vaginal barrel ( inner most part ) balloons (imgaine a vase with a large base) and this is something you'll feel when you insert your penis all the way into the vagina (close to the end of the vagainal barrel you notice your knob is in a cavity) when she finally reaches (orgasmos) her vaginal barrel muscles start to contract spasmodically and involuntarily thats when they grip the sheets also, she arch her back (for some some strange and unknow reason they try to stop it) and all there mind is focused on what is about to happen, its at that point I closed my eyes( because of the intense pressure of the contracting kegals muscles around my penis) and shot my entire load - the unbearable intense pleasure just about lifts your head off !! So I guess both of you are experiecning this highly intense pleasure It reminded me at the time of being born again as strange as that sounds, thinking about it this probably evokes feelings of being born again and the bond you have with your mother and the mothers bond with the child. I guess its vey similar in a lot of ways. Also your penis no longer feels part of you, you and your women feel like one and the same, its a fusion of your souls, locked together in time, I would go as far as saying its like an out of body experience. The reason I say this is because when its over its suddenly so quiet and peaceful and the feeling of euphoria is amazing, two neuro-chemicals are released vasopressin and oxytocin.