Bread baskets (also known as banneton baskets) are a must-have for bakers. They are used during the proofing stage of making sourdough bread, and they help prevent the loaf from spreading as it rises, so that it's in the shape you want when you transfer it to your baking vessel. Source
amishbaskets.com
Several types of baskets are available, including those made of natural materials like rattan and cane and those made from synthetic materials, such as pollyrattan. Some are lined with removable fabric, so you can easily remove the dough from the basket after it has risen.
When choosing a proofing basket, look for one that's the right size and has the capacity to hold the weight of your loaf. A typical 8-inch round or oval banneton will accommodate about 1 pound of dough, while a 10-inch basket is able to hold 2 pounds of bread.
Choosing the Right Style and Size for Your Freshly Baked Loaves
If you don't have a bread basket, use a bowl lined with a flour-dusted lint-free kitchen towel as a substitute. This will work for most round loaves, but if you're baking an
elongated ring-shaped loaf (as with a Dutch oven), it's better to use a basket.
For the best results, make sure you give your new proofing basket a good dusting with flour before using it to ensure that your dough doesn't stick to the rattan pieces. Then, once the basket has dried, lightly flour it again before every use to keep it looking its best and to ensure that your dough doesn't stick in places where it shouldn't.