A 3-way switch is a type of electrical switch that allows for the control of a single light fixture from two different locations. This is typically used in hallways, stairwells, and other areas where it is convenient to have multiple points of control for a light.
3-way switches are more complex than single-pole switches, but they are not difficult to install. With a little bit of electrical knowledge and some basic tools, you can easily wire a 3-way switch yourself.