A function is any equation where for any input in the equation, the output yields exactly one value. Typically, the input is called x and the output is called y.
Functions are best understood by examples. First, consider the equation y = x?. Given an x, the square of x only yields one value for y. This is a function.
Next, consider the equation y? = x. Given an x, there can be two possibilities for y. If x is 4, y can be either positive or negative 2. This is not a function, since y can be more than one value with a given x.