Kombu dashi (or konbu dashi) is a simple, but important stock used widely in Japanese cooking, for soups, stews, noodle dishes and the like. You can also make dashi, or stock, from other types of savory dried items, like mushrooms, katsuobushi (dried fish flakes) and niboshi (dried anchovies/sardines). There is kombu (a thick, dried kelp) that is available specifically for making dashi called kombu dashi.
The key for making good dashi is to not overboil the kombu.
6 cups of cold water
1 strip of dried kelp, about 5-6" inches (kombu こんぶ)
Soak the kombu in 6 cups of water in a pot for an hour or two (or overnight if you have the time)
Put the kombu and water on medium heat, bringing the liquid to a simmer (do not bring it to a boil). Remove the kombu -- you can save this kombu to use in other dishes. You can save the dashi for later use -- just refrigerate (not more than 1 week).
Alternative method: just to soak the kombu in cold water for several hours or overnight in the refrigerator, without boiling it. If you use this method, simply strain the kombu dashi and use. You can reuse the kombu 2x more this way.