litres+or+milliliters+or+millimeters

I am having trouble making clear to myself or to my 12 year old students the units that are used for rainfall.

A climate graph speaks about millimeters per month of rain. Let's say 100mm.

How many litres would that be? And is that litres per hour, or liters per square meter, or what?

I'm from America, by the way, and there we just talk about inches of rain!