What is Rain Rate in mm/hour?

Very simply, it means that a container would collect rain up to a depth of X mm of rain water in one hour.  It also means that a field with an area of one hectare would also collect the same depth of X mm of rain water in one hour.

The volume of rain water collected would be the area multiplied by the depth of water collected.  Thus, one hectared with an area of 10,000 square meters would collect a volume of water equivalent to 10,000 multiplied by the rain rate in meters, or X (mm) divided by 1000 (mm/meter).  If the rain rate is 50 mm/hour, the volume of rain falling on one hectare of area would be 10,000 * (50 / 1000), or 500 cubic meters per hour.

Now 500 cubic meters per hour should not be a cause of concern if the drainage system has a capacity of 500 cubic meters per hour or more.  However, it becomes significant if the drainage capacity is only 100 cubic meters per hour, because the area will be accumulating 400 cubic meters of water per hour more than it can safely convey elsewhere.

The result would be flooding to the degree of the amount of rain rate.  In the example above, a rain rate of 10 mm/hour would accumulate only 100 cubic meters per hour, which would be fine because the drainage system has sufficient capacity to safely convey 100 cubic meters per hour.