Annex 48 is a group of people who are studying how to make things called heat pumps and reversible air conditioners work better. Do you know what an air conditioner does? It makes a room cooler when it's hot outside. A heat pump can do that too, but it can also make a room warmer when it's cold outside.
Think of a heat pump like a magic box. It takes warmth from outside and puts it inside your house. It's like bringing a cozy blanket inside on a cold day. If it gets too hot outside, the heat pump can take the warmth from inside your home and send it outside. It's like wearing shorts on a hot day.
But sometimes, heat pumps and air conditioners don't work very well. That's where the Annex 48 group comes in. They figure out ways to make the magic box work even better. Maybe they can make it use less electricity or make it work faster. They are trying to make the magic box work better so you can be more comfortable in your home, no matter what the weather is like outside.