Multi-room audio, put simply, is the ability to listen to music in several rooms. Depending on the system you choose, you'll be able to play different music in different rooms at the same time, or the same music in all rooms, or just a couple simultaneously.
Music played across a multi-room audio system can be from streaming services your own personal music collection whether that be on a network drive, your computer, a CD or even vinyl - with the right setup. Multi-room audio systems tend to be wireless and controlled using some sort of app, so aside from ensuring a power supply is nearby and you have your smartphone ready, speakers can be positioned wherever you like in your home. Naturally, you'll need at least one speaker in each room you want to hear music in.