When your in low gear what sprocket are you using?
It depends on whether you are talking about the front sprocket
(the one at the pedals, called a chainring) or the sprocket at the
rear derailleur.
The lowest gear at the front is the smallest sprocket/chainring.
The lowest gear at the rear is the largest sprocket. So if you
combine the smallest sprocket at the front with the largest
sprocket in the rear you have the lowest gear available on your
bike.