Do what it says, one change gear on the stud, the other on the lead screw. Shows the resulting thread pitch. Intermediate gears do not change the gear ratio, as long as they are not compound gears. Intermediate gears do change the direction of rotation.
This pic shows a double gear in the change gear position, make sure both the other gears are using the same gear, as said already, the number of teeth do not matter on that gear. Both gears are used in compounding, namely for doing metric gears. Do not worry about that for now.
Google the "How to run a lathe", there is PDF copies all over the internet. Or watch E-bay for the actual books.