A mosh pit is an area at the front of the crowd, a few rows back from the front row typically, where young males (mostly) engage in a form of dance that involves shoving and ramming, aka slam dancing. There is typically enough room for people to move around in a circle -- imagine a tornado of young men with shaved heads or mohawks -- and gain enough momentum to knock each other down. It's not for the faint at heart.
The crush of fans at a concert's front row isn't necessarily a mosh pit, although you can certainly get "mashed." I once saw the Minneapolis rock group The Replacements a club in Cincinnati called Bogart's. I was at the very front, and was pushed up against the wooden wall in front of me all night. By the end of the concert I felt like I had been in a sumo match with a hippo and lost. But the moshing going on behind me all night was far worse I'm sure.
