A pass refers to the catch of the striker stick in the empty hand, the release of the original handle, and the exchange of the nunchaku to the other hand. A pass-stance is the position of your body when you have a stick in each hand.
If you catch the striker in the same grip you have the handle, it's a slick pass (FF or BB). If you catch the striker in a different grip to the handle, it's a gritch (grip-switch) pass (FB or BF).
A mirror-pass is the transition from one pass-stance to the mirrored version, for example a left shoulder stance to a right shoulder stance.
Passes can be divided roughly by the plane they swing on when they're caught. Passes in the same plane tend to flow together well.