Definition - What does AirPlay mean?
AirPlay is a wireless streaming service from Apple designed to allow users to stream content from iDevices and Macs. AirPlay is designed to take a user's iTunes multimedia content from the device on which it's hosted and make it available to multiple other sources, including supported speakers/stereo systems, an AirPort Express or an HDTV set.
Techopedia explains AirPlay
The streaming content service works primarily within Apple's own ecosystem or products that support it. The service is based on multimedia - video, audio, photos and apps - being transfered between systems and devices. The content is streamed wirelessly from the hosted system, like an iPad or a Mac, to an HDTV that has an Apple TV connected to it.
You can stream iTunes content from either your Mac or your iDevice to HDTV sets with an Apple TV attached to them, any speakers attached to an AirPort Express Wi-Fi base station, and specific AirPlay-enabled hardware, like AirPlay-supported speakers.
There is also a feature within AirPlay called AirPlay Mirroring. It allows users to see their iDevice screen on a big screen when connecting to an Apple TV. Users can play games, for instance, and enjoy them on the big screen where the whole family can see them, instead of just on a small iPhone display.