This article pertains to motorcycles with "manual choke". If you have a fuel injected bike most probably you do not have a manual choke as this operation is controlled by the onboard computer.
When the motorcycle engine is cold, the use of the choke facilitates the start up and warming of the engine allowing a richer mix (more fuel) to enter the engine.
Explanation: when opening the "choke" in reality you are closing (choking) the air intake thus changing the proportion of the mixture that reaches the engine, making it a lot richer to allow an easy start. As more gas is entering the engine more wear and tear is generated by the gas washing away the oil in the cylinder walls.
The choke lever may be located in the left side of the handlebar by the grip, at the carburetor itself or at your instrument panel depending on the make and model.
Is very important that you use your choke "the minimum time possible", as the extended use of the choke will damage the longevity of your engine. NEVER forget the choke on and take of!
Usually a couple of minutes with the choke on is all you need, then just wait till your engine warms up thoroughly and you are ready for riding.