min is the minimum time before anything happens
max is the maximum time
the game will choose a time between the min and max, but probably close to the mid.
So in your example the time would usually be around 50s, but it could be anywhere in the range. Normally you would put mid halfway between min and max, but sometimes you want to bias it one way or the other.
Note that in triggers, countdown is a delay between activation/condition being met and the On activation field operating. In waypoints this is called Timeout. However, in triggers, timeout means that the condition has to be met for that length of time before the trigger fires.
So, in a trigger, if you have countdown of 10s then the trigger will fire after 10s. It's the same in a WP, 10s after the waypoint conditions are satisfied, the on activation will happen.
If you have timeout of 10s in a trigger then the condition (eg west present) has to be met for 10s before the on activation field operates.