A receiver is that big, heavy thing that you plug your speakers and other components into (like a DVD player, TV, CD player, Xbox, PlayStation, iPod, and etc.). It's the "brain" of the show, really. The idea of connecting all your components to a receiver is the concept of audio/video switching, allowing you to switch to different video sources (like TV, DVD, camcorder) on your TV and thus changing the audio source accordingly - all without touching anything but the receiver.
It's the "brain" of the show, really.
Of course, the main purpose behind audio/video switching with a receiver is to drive audio to external speakers, like surround sound or stereo speakers.