What happens as different values of x are plugged in?
When different values of x are plugged in f(x) i.e. domain of the function, it gives out the values of x called range of the function.
As x increases, does f(x) increase? Does it decrease?
It depends upon the function whether it is increasing or decreasing. If the function is increasing, so by increasing x the value of the function (i.e. range) will be increased. Similarly, if the function is decreasing, so by increasing x the value of the function (i.e. range) will be decreasing (Marsden, Jerrold & Alan, 1985).