Division by zero is an operation for which you cannot find an answer, so it is disallowed or undefined. You can understand this if you think about how division and multiplication are related. But no value would work for x because 0 times any number is 0. So division by zero doesn't work.
The expression of a number divided by zero has no meaning, as there is no number which, when multiplied by 0, gives a (assuming a ≠ 0), and so division by zero is undefined. Since any number multiplied by zero is zero, the expression 0/0 is also undefined; when it is the form of a limit, it is an indeterminate form.
a x 0 = 0