LED lights must be driven with a constant current or voltage source. When you reduce the forward current or voltage across an LED it will dim slightly and then quickly drop off to zero light output.

Dimming drivers can dim LED lights with Pulse Width Modulation (PWM). Pulse Width Modulation is a method of transmitting information on a series of pulses. PWM can control the pulse width and duty cycle causing an LED light to vary its intensity. This works by turning the current off for an extremely short period of time, which will in effect turn of the LED.

The frequency could range from a hundred modulations per second to as high as hundreds of thousands of modulations per second; this makes the LED appear to be continuously lighted without any flickering. To produce an increased dimming effect, the LED will remain off for a longer period of time per duty cycle so the smaller the duty cycle the dimmer the light output. The eye will visually detect the LED as a continuous stream of light that only appears dimmer.
 

LED dimming does not result in a loss of efficiency. During dimming, the LED lights are still operated at the same voltage and current as during full light output. Dimming LED lights will not affect the already long life expectancy; it may even increase the life expectancy due to the reduction of the operating temperature of the LED.

The fitting itself cannot be dimmed by modulating the current level, rather by modulating the duty cycle. An external controller or driver usually does this.