What does a dimmer reduce volts or amps?

Answer 1

Voltage. A dimmer usaly chops the sine wave, leting it pass after a while.

Another answerThe dimmer effectively becomes another device in the circuit. It effectively becomes a series circuit, therefore the voltage to the dimmed lights will be reduced in proportion with the relative position of the dimmer switch.

HOWEVER...

there is such a thing as "pulse width modulation" dimmers. They are typically used for devices that need full voltage but but you need to reduce output... such as fluorescent lights some fans and many other motors.

Most dimmers are resistance type dimmers since the pulse width modulation dimmers are far more expensive.

Answer 2

A dimmer reduces the voltage supplied to a light fixture, which in turn decreases the intensity of the light. It does not directly reduce the amperage, but the decrease in voltage will result in a lower current flowing through the circuit.