if your y value is increasing as your x value is increasing, the function is increasing.
for example f(x) = x
if x = 0, y = 0
if x = 1, y = 1
So as you go from left to right, your y value gets higher, thus it is increasing.
I've attached some graphs of increasing functions.
(I feel like a kid with candy)
edit: Note that is not increasing at the point (0,0), but it is increasing everywhere else.