Moshing basically is an expression of music by the crowd at a concert/gig. It occurs when bands usually of the rock, punk and metal genres play in a live setting. Now it's difficult to explain the essence of the mosh pit without completely underplaying it. But I'll try. At these gigs, members of the crowd will gather at the front and will furiously and passionately shove/ push/ body slam each other in time to the music. But the idea is not to cause any harm to each other( even though that's usually the case ) but just to enjoy the music in a less passive way, like you would if you were to dance to any type of music. But its better than just a dance , much better.