Couche is a heavy linen fabric, and it is used to proof shaped loaves on top. Baguettes are usually proofed on couche, with folded couche in between to create support. Boules and batards can be proofed on couche, as long as the dough is strong enough. Boules and batards made with a soft dough should proof in baskets, as they create more support for the dough's structure.
Couche also makes it easy to transfer loaves when baking. Fully proofed loaves are not to be touched directly by hand, as it can leave finger marks and deflate the dough. By having loaves proofing on couche, you can turn and move your loaf without damaging it.
Couche should not be washed. After using, hang dry and scrape off extra flour. It is normal to have darkening of color after a long usage, especially if the couche is used for retarded dough.
At SFBI, we use 26" width couche which fits to the length of proofing boards. For home bakers who are not baking full-length baguettes, 18" width couche may be easier to use.