In an engine, the throttle is the mechanism by which the engine's power is increased or decreased. Throttle may refer both to the part inside the carburetor (the throttle slide) which regulates the mixture of air and fuel which enters the engine, or to the control which indirectly controls the engine's power.
By convention, moped throttle controls are on the right end of the handlebars. Twisting back on this grip increases the engine's RPMs. As most mopeds have automatic transmissions, twisting the grip should cause the clutch to engage and start the bike moving.
The throttle controls accomplish their job by means of a throttle cable, which is attached to the controls on the handlebars, which pulls up the throttle slide in the carburetor, allowing greater amounts of gas and fuel into the engine's intake.