In a car park, it's a double space, one behind another.
You can drive through the first so that you are facing out at the second.
Saves having to reverse in or come out backwards.
She was looking for a space in the car park when she noticed a throughbie, so she drove straight in and was set to leave without having to reverse into traffic.
Female: Hey babe what are you doing tonight?
Male: Well I've been having a bad day so I think to make myself feel better, I'm going to come on over and throughthegate you.