A Lilo is a rubber (or plastic) mattress you fill with air and use as a bed, or float on in a pool or the sea. In British English especially, Lilo began as a brand (written Li-Lo, from the 1940s to the 1970s) and then became an ordinary word for that kind of air-bed — the way some people say a brand name for a whole type of thing. American English more often says air mattress. Though actually in Taiwan almost nobody calls it a Lilo; they will look blank unless you say the Chinese for air mattress. The trademark was a catchy spelling of lie low, because you lie low on it. It has nothing to do with the girl in the cartoon Lilo & Stitch.