Cylinder is the place where the fuel-air mixture is compressed and burned to convert chemical energy to heat and mechanical energy to make the bike go. Just like in any other internal combustion engine. The engine might have only one cylinder or many. More than four is rare, though.The cylinder is fired by a spark plug which creates a spark electrically from the battery .
It is the cylinder in an engine where the combustion takes place-..in vehicle the cylinder is just above the transmission. think of it as a syringe. the piston moves up and compresses the air fuel mixture and as a result combustion takes place. the piston is then pushed down. this up and down motion of the piston rotates the crankshaft which is connected to the transmission which transmits power to the wheels. there are various configurations of cylinders in an engine. in vehicle have single cylinder, parallel twin, v twin, L twin, inline 4, inline 3, inline 6, v4, flat twin. similarly in cars you get inline 3, inline4, inline6 , v6, v8, v10, w12. the number represents the no. of cylinders and the letter or prefix represents the configuration.