Can somebody explain a function to me?

A function is a relation in which each input has only one output.

In the relation, y is a function of x, because for each input x (1, 2, 3, or 0), there is only one output y. x is not a function of y, because the input y = 3 has multiple outputs: x = 1 and x = 2.

Step-by-step explanation:

In mathematics, a function is a relation between sets that associates to every element of a first set exactly one element of the second set ... The symbol that is used for representing the input is the variable of the function (one often says that f is a function of the variable x).