Reefing is a way to make a big sail on a boat smaller so that it won't catch as much wind. It's like when you put on a jacket to stay warm in winter, but if you get too hot, you take off the jacket or unzip it a little bit to cool down.
When you're sailing on a boat and the wind is really strong, you might need to make the sail smaller so you don't tip over or go too fast. Reefing lets you adjust the size of the sail by folding or tying it up in a special way.
On some boats, there are lines and hooks along the sail that you use to reef it. You pull on the lines to make the sail fold up and then you use the hooks to keep it in place. It's kind of like folding a piece of paper and then clipping it together so it doesn't come undone.
Reefing can be important because it helps keep the boat under control and makes sure you don't go too fast. It's really important to know when and how to reef if you're sailing a boat so that you can stay safe and have fun!