Torque in a motorcycle
The piston moves up and down, and the force for that comes from the fuel that is burned. Connected to the piston is a rod, the connecting rod, and that rod is connected (with the ability to turn) to the crankshaft.
The distance between the pedal to the rotation point is comparable to the distance between the crank and the middle point of the crankshaft, and in the same way as in the bicycle example, the distance "counts" for 100% when the point of the crank that is connected to the rod has an angle of 90 degrees with the direction of the piston, and for 0% when that point is in the highest or the lowest position.
A piston, therefore, delivers a varying amount of torque. The torque at a certain rpm is the average torque that the piston delivers during the revolution stroke. And when your motorcycle has more than one cylinder, the torque of the individual pistons add up.
The rpm where the maximum torque is delivered, is the rpm where the fuel is burned most efficiently: it is at that rpm that the piston delivers the maximum torque on the crank.
The gearbox transmits the rotation of the crank onto (ultimately) the wheels.
What does power (BHP) mean?
The amount of power tells how fast work can be done. When you walk up the stairs for instance, you deliver work, carrying your own weight up the stairs. How fast you can do that determines your power (in combination with the length of the stairs and your weight: they determine the amount of work).
Power is expressed in kilowatt (Kw) or in Horsepower (Hp).
When you know the torque of your motorcycle with a given rpm, you can deduce the power, by multiplying the torque with the number of revolutions per minute (and of course with a constant factor to adjust the different dimensions like rpm, Hp and Nm).