DEFINITIONS:
A function $\,f\,$ increases on an interval $\,I\,$
if and only if
whenever $\,a < b\,$ in $\,I\,$, $\,f(a) < f(b)\,$.


A function $\,f\,$ decreases on an interval $\,I\,$
if and only if
whenever $\,a < b\,$ in $\,I\,$, $\,f(a) > f(b)\,$.


Roughly:
increasing means going strictly UPHILL, as you move from left to right on a graph
decreasing means going strictly DOWNHILL, as you move from left to right on a graph