Rainfall intensity is the measure of how "hard" it is raining. The harder it rains, the higher the intensity. Intensity is defined as the volume of rainfall that falls for a given time period divided by that time. For any given rainfall storm event, on average the longer the storm lasts, the lower the overall intensity will be.

This is consistent with what we would intuitively expect. Any given storm may rain hard for a short period of time, but it builds to that intensity and falls from that intensity over a period of time.