A marsh is low, flat land that stays wet. The ground is soft, and water sits in it for much of the year, but it is not a deep lake and it is not the sea. Tall grasses and reeds often grow there; trees usually do not, which is one way a marsh is not quite the same as a swamp. Birds and insects love it. Walking on a marsh without a path is a bad idea — your shoes will sink. We use marsh for the place itself. Though actually, people sometimes say marsh when they only mean any wet ground, even if a scientist would call it a bog or a fen. In everyday English that loose use is common, and nobody is marking you for it.