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It's a little cheesecloth or muslin bag with a drawstring top, or a piece of cheesecloth you tie with string, in which to put spices (such as a bouquet garni) that you will be immersing in cooking liquid. You can later remove the spice bag; the object is not to have peppercorns and stuff randomly floating around.
I'd think it's just a little muslin or mesh bag or sachet filled with various spices - kind of like a tea bag, that you can easily extract from your pot or other cooking vessel after the spices have infused whatever you're making. Not sure if that helps?
Rachael is a trusted home cook.
I think both of the above answers nailed it. A great substitute if you don't have one is diaper (burp) cloths. I don't use them with the kids anymore, but I kept them because they're about the same thickness as cheesecloth. I just cut them up with scissors and, as long as it wasn't used in tomato sauce or beet juice, you can just throw them in the washer and reuse them.
Ain't no thing.
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