Future preterite can be described as the future seen from the past.
It is used when speaking about something you were planning at a point of time in the past, or something you thought was going to happen. The forms are the same as for the conditional, skulle + infinitive, but the meaning is not the same.
Example:
När vi skulle åka upptäckte vi att bilen var stulen .
When we were going to go, we discovered that the car was stolen.
The people in this sentence discovered that their car was stolen at a point in time where their leaving had not yet happened – they were going to go, the leaving was still in the future for them.