Parchment paper is something I could do without, but I’d much rather have it. It’s a specially treated sheet of paper that is almost non-stick. Bakers use parchment paper in two ways.
As a surface to rise dough on, then transfer it to the oven. The loaves then bake on the parchment paper.
As a liner for brioche, kugelhupf, cupcake and other dough molds.
Parchment paper is available in rolls, as pre-cut circles or as large sheets. I bought a 1,000 sheet box of the large sheets several years ago and I haven’t used even a third yet, even though my wife and I bake a lot. I found the rolls curled up and were expensive to boot. I’ve never tried the pre-cut shapes, but I can’t see how they could be better than the sheets — certainly not cheaper.
There is a downside to parchment paper, however. It will scorch above about 475-500F / 245-260C and will char badly at 550F / 287C, which is where I bake a lot of my pitas and pizza. The up side is that if you are baking at lower temperatures, 350-375F / 175-190C, you can reuse the paper at least once, and maybe several times.