The S drive gear for a Honda Civic, this is a mode you will want to be in when You are driving through difficult terrain as it will give You better engine braking ability and better acceleration in which the transmission downshifts sooner when you open the throttle to pass and holds lower gears longer for sustained acceleration.
This causes to shift the transmission to lower range of gear ratio where you can reach higher RPM’s you get better acceleration but compromise on mileage. Use this If you wanna pass someone etc or like sports mode.
The L drive gear will stand for Low. This mode will give You more torque and stronger engine power so that You can make it up large hills. This mode could be good for some more desolate areas in which You will be gaining elevation as You go down the highway. It locks the gear in low max second.