The countdown fields of a trigger mean that there will be a delay between the trigger being fired (i.e. condition field satisfied), and the consequences occurring (i.e. on activation field exectuting.)
The trigger waits for a random time: that time is more than min, less than max, and likely to be somewhere near mid.
If you toggle the field to Timeout instead of countdown, then it means that the trigger condition must be satisfied for that time before it fires. This is rarely used, whereas Countdown is very common.
Confusingly, Waypoints have a field called Timeout which in fact operates like the Countdown in triggers. At least I think it's all that way round.
Trigger countdown example:
Countdown 3 20 10
Condition: testVar
On Activation: hint "gotcha!"
When the variable testVar becomes true, a hint saying "gotcha" will appear. The delay between the variable going true and the hint appearing will always be between 3 and 20 seconds, and will usually be around 10s.