Vulcanized fiber is a special kind of material made from wood pulp (the stuff that paper is made of) that has undergone a process called vulcanization.
See, when you make something out of wood pulp, it's kind of like using glue to stick lots of tiny wood fibers together to make a sheet of paper. But if you try to use that paper-like material for lots of different things (like making parts for machines, or even things like suitcases and some musical instruments), it can be too flimsy and bendy and not very strong.
That's where vulcanization comes in!
The vulcanization process takes the wood pulp (which is already in a sheet-like form) and adds chemicals to it. This makes the material much stronger and more durable than regular wood pulp or paper. It also makes vulcanized fiber resistant to things like water, chemicals, and electricity.
The final material that comes out of this process looks and feels a lot like paper, but it's stronger, denser, and more rigid. It can be molded and shaped into all kinds of useful things, like gears and washers for machines, or parts for musical instruments like accordions and tambourines.
So, in short: Vulcanized fiber is a really strong and tough kind of material made from wood pulp that has been treated with special chemicals to make it more useful for lots of different purposes.