Stays are like big ropes that help keep a sailboat's mast standing up straight. The mast is the tall pole that holds up the sails on a boat, but it needs help to stay upright in the wind and waves. The stays are kind of like big, strong strings that are attached to the top of the mast and then go down to the sides of the boat.
Imagine if you were building a tall block tower out of LEGOs. If the tower gets really high, it might start to wobble and fall over. But if you had some long, skinny sticks that you could attach to the top of the tower and lean against the sides, the tower would become much more stable and wouldn't fall over so easily. That's kind of like what stays do for a sailboat mast!
There are different kinds of stays depending on where they attach to the mast and where they connect to the boat. Some stays are attached to the front of the mast and go to the front of the boat, while others connect to the sides and go to the back. But all of them work together to help keep the mast upright so that the sails can catch the wind and propel the boat forward.