Kelp is a large brown seaweed — a plant that grows in cold, fairly shallow sea water and stands up from the sea floor in long, flat ribbons. Some kinds grow so tall they make underwater 'forests', where fish and other animals hide. People cut kelp for food (the thick dark sheets in soup are often this), for iodine, and for a jelly-like stuff used to thicken ice cream, sauces, and toothpaste. In everyday English it is a plain nature-and-food word, not slang. Scientists also use kelp as a group name for many big brown seaweeds; though actually, a packet in a shop labelled 'kelp' is usually just one edible kind, not the whole group. You will meet it in food, geography, and talk about the sea and climate, because those forests take in a lot of carbon.