AirPlay gives the ability to play content from an iOS device wirelessly on another enabled device. AirPlay falls into 2 broad categories. Video, which is handled exclusively by AppleTV, and audio, which is gathering steam as one of the standards which must be built into hifi and home theatre hardware. The simplest connection possible is to take the HDMI output from AppleTV to the HDTV, and play audio through the TV speakers, but connection may also be made to an AV receiver, which can process the video and sound before passing it to the TV and speakers. Using AppleTV, one can mirror the display of an iOS device, and play media held on such a device without making a physical connection. All that is required is that the AppleTV and iOS device are on the same network. Audio may also be passed from an iOS or OS X device to a suitably equipped amplifier or set of powered speakers. Check with the manufacturer which models are AirPlay equipped. Apple's website showcases a huge array of speakers ranging in price from $50 to over $1000. Check out available offerings here.