it is the cylinder in an engine where the combustion takes bikes 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 bikes you 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.