It's Natural Gas that has been cooled to the point that it becomes a liquid, hence, Liquefied Natural Gas. It's typically kept in heavily insulated containers or tanks, and is the primary method of long-distance transport of gas other than pipelines. In its liquid form it can be put on ships and transported somewhat like crude oil, but it needs to be kept cold, and once it's at the terminal it needs to be turned back into a gas to be used.