There was a time when modern audio equipment was mostly rack mounted, when they were built out of a stack of components: one receiver, one cassette deck, one amplifier, a record player, and maybe even a reel to reel deck. Those were the days of "Big is Beautiful"
The victim of neglect in this restoration video is one of those parts: a control amplifier where all inputs come together, and you can select what to listen to. Usually this would be followed by either a built in end amplifier, or a separate one.