"Division is one of the four basic operations of arithmetic, the others being addition, subtraction, and multiplication. The division of two natural numbers is the process of calculating the number of times one number is contained within another one." This is from the Wikipedia page on division.
Basically, you split the whole number into equal parts. Imagine fair sharing. Each person gets the same amount. This is the same concept.
Division is the opposite of multiplying so if 4 x 3 = 12, then 12 / 3 = 4