How do I determine my bike's fork width?
This refers to the interior distance between the rear wheel dropouts on the frame. The rear triangle is sometimes referred to as the frame 'fork', not to be confused with the front wheel fork.

The vast majority of modern bikes with quick release hubs have 100mm spaced front wheel dropouts and either 130 or 135mm rear wheel dropouts. If you measure the distance with the wheel out it may be a few mm off of 130 or 135mm as frames flex a bit when the quick release is undone.