Okay, so you know how sailboats have big pieces of fabric that catch the wind and make the boat go? Those are called sails. Now, sometimes people like to race their sailboats and make sure everyone is using the same type of sail so it's fair. To help with this, they have different markings on the sail.
The markings tell you what class the sailboat is in and what type of sail it has. For example, the letter "A" might mean it's a bigger sail than the letter "B". This way, everyone knows what type of sail everyone else is using and can race fairly.
It's kind of like how in a game, everyone agrees to play with the same rules and use the same equipment. The sail class markings make sure everyone is really playing with the same rules, even if they have different boats.