Engines have pistons that compress then decompress in an engine and whenever the piston compress it is actually compressing a type of fuel (idk what it's called) and the spark plug of that piston sparks and ignites the fuel. This launches the piston to the other side where another spark hits and it returns. The sound of a motorcycle running is actually all those sparks going off and it's just that some motorcycles have more sparks/more powerful spark plugs.