It's a Variac testing instrument. What does it do? Well, I'm not really sure. I've had it for years collecting dust on my workbench. Here's what the internet told me:
This is a vintage Variac variable transformer with a 115V input and adjustable output voltage ranging from 0V to approximately 130V, as indicated on the dial. It is designed for precise AC voltage control, making it ideal for applications such as audio equipment testing, tube amplifiers, and laboratory use.
The best way for me to describe it is that it is basically one big, heavy rheostat. You can plug this into the wall, then plug a tool like a drill into it, then you can adjust the speed of the tool by turning the dial. I'm sure that's not its intended use, but that's the easiest way to describe it.
What I do know is that they are going for anywhere from $130 to $200 on Ebay, so I figure $50 is a good asking price. This is probably something that nobody uses these days, but it's really a cool vintage item that might look great as a display or something.