No issues with you asking questions. I'll try to address your question in non-car-speak: The difference is that a boxer engine is still like a traditional engine, where you have # of cylinders = # of pistons. In this engine you have 2x the number of pistons in each cylinder.
In a traditional IC engine, each piston moves in and out of each cylinder - and in your head, you can imagine the cylinder as being like a "cup" that the piston moves into and out of. That's how a traditional IC engine, boxer or not, works.
In this engine, instead of the cylinder being shaped like a "cup" that a piston slides into, the cylinder is actually a real cylinder that is open at both ends. And there are two pistons that move in that one cylinder.
Hope that helps.