Keeping your home's pool heated is typically necessary even during the warmest of summer months, as water in a pool can easily lose heat as it evaporates. While you don't want to keep the pool water so warm that you encourage algae to grow, you also don't want it so cold that you can't enjoy your pool! To keep the pool's water warm, you may have a heater attached to the pump so that water pulled through the filter is heated before it's sent back to the pool, but note a few other pool heating tips; this will keep the pool comfortable and also cut down on wear and tear of the heater itself.
When wind blows across the surface of your pool's water, it will cool the water off—much the same way you cool a bowl of soup by blowing on it! Adding wind shields on the sides of the pool can help to trap that otherwise lost heat; there are shields that are purposely made to protect the pool and which may arch over the surface of it, or you can have glass fence panels installed around the pool to stop wind and keep heat in the water.
Liquid blankets are, as the name implies, a type of liquid that is sold in bottles, and which is meant for pools in particular. You simply pour this liquid over the surface of the pool, and it then forms a layer on the water's surface. This layer is not so thick that you can actually feel it against your skin, but it is thick enough to trap some heat in the water. This type of liquid blanket also keeps water from evaporating so that you don't need to refill the pool as often as you would otherwise.
There is a difference between a pool covering and a pool blanket; a pool covering is meant to protect the water from debris that could clog the filter or settle at the bottom of the pool. This covering might be very lightweight and thin, and may do little to actually retain heat in the water. A pool blanket, however, does trap heat in the water, just like a standard blanket. It may not do much to keep the water warm once you pull the blanket away to swim, but you won't need to run the pool heater as much as you would otherwise in order to make the water comfortable for when you are ready to get into the pool.