Well first, an interval is a set of real numbers, usually x values. For example 0 ≤ x ≤ 1 is an interval. If the y values for the x values of the set increase as the x values increase, then the interval is increasing. If the y values for the x values of the set decrease as the x values increase, then the interval is decreasing. I hope that wasn't confusing, if you have questions, ask.
