Three way switches are used to have two switches turn on light(s). They can do that because there is always power to both switches even if one of the switches is turned off. This article teaches you how to wire a three way switch.
brief how to
A three way switch has 4 screw terminals:
The First Box: The First box has the power come into it with a 2 wire cable, and a 3 wire cable going out if it to the second box and switch.
The Second Box: The second box has the 3 wire cable coming in from the first box and a 2 wire cable going out to the light.
The Traveler Wire: The 3 wire cable that goes between boxes is called the "Traveler". The 3 wire cable allows power to go to the second switch even when the first switch is turned off.
Attach the 3 wire traveler cable coming from the power source to the terminals on the first switch:
Attach the 2 wire cable from the power source to the screw on the second switch.
Push the wires into the box and screw the switch to the box.
Attach the 3 wire traveler cable coming from the first box to the terminals on the second switch:
Attach the 2 wire cable going to the lights to the screws on the second switch.
After the connections are made put the wires into the box and screw the switch to the box. Use wire nuts to
Attach the light to the 2 wire cable. Black to Black, White to White, and Ground to Ground. Push the wires into the ceiling box and attach the light fixture to it.
Turn the breaker on and test your 3 way light switches.