A daybed is a square piece of foam that sits on a low platform and is usually made up with a colorful cover and pillows, and sometimes a backrest and even a canopy, though most daybeds aren't shaded. (For that, you need a cabana.)
They're generally big for a couple, perfect for three people, and four or even five can squeeze onto one.
Daybeds can be placed at water's edge, or in front of the cabanas, or anywhere outside of heavy traffic. They sometimes come with a first drink or two included.
Prices for daybeds, or course, depend on the pool/dayclub, but they generally start at $50 weekdays and go up to $150 (or more) weekends.