Type 1 font
A Type 1 font is the older of the two shapes a font file comes in.
In more detail
It was the shape professional typesetting was built on for years, and a great many fonts were cut for it and never redrawn. A firm whose house font dates from that time often still holds it in exactly that form.
The file is either one piece or three, depending on how it was delivered; both are read the same way and mean the same thing.
What it does on a real document
A document carries the file whole, exactly as it was handed over, so the letters come out as their designer cut them.
A request hands the file over beside its description and names it, and every piece of text set in it refers to that name.
Where it comes from, and why
It was published in the mid nineteen eighties, and for a long while the details of how it worked were held back, which is a good part of why a free alternative was drawn up.
The details were released in 1990, and the format is still read today, which is what lets a font cut forty years ago set a page made this morning.