Most stereo and home theater receivers have a Speaker A and Speaker B switch on the front panel. Speaker A is used for the speakers in the main room and Speaker B is intended for a second pair of speakers in another room or the patio. Taking advantage of this feature requires running speaker wires from the receiver to the second room and connecting a second pair of speakers. Most receivers can power speakers in both rooms (Speakers A & B) at the same time; other receivers permit only one pair of speakers at a time. Refer to the receiver's owner's manual for more information.
It is also possible to use the Speaker B switch to power more than one additional pair of speakers, but a special speaker switch is required to do this safely. The speaker switch required has an 'impedance matching' feature that protects the receiver from damage caused by powering too many speakers at once. A speaker switch with impedance matching can be purchased for various prices here and here. It's a great way to use your receiver for a basic multiroom audio system.