What is kombu?
Kombu is Japanese dried kelp and it's used to make dashi, which is the base broth for miso. http://www.edenfoods.com/store/product_info.php?products_id=108880
Adding to the excellent answer from MirandaR, kombu is thick and leathery, unlike most seaweeds you'll see. Another use for it is to add it when cooking dry beans. It's supposed to make them more digestible (less gas inciting). You should be able to find it in most health food stores -- if not, as them to order it for you.