A classification society is like a big group of grown-ups who make rules for really big boats and check to make sure they follow the rules.
Just like grown-ups have rules for things like crossing the street or using sharp objects, classification societies have rules for how big boats are built, how they work, and how they stay safe in the water.
But the grown-ups in a classification society don't just make rules and then walk away. They also check on boats to make sure they are following the rules. They might look at things like the materials the boat is made of or how it's been put together. They also check safety features, like lifeboats or fire prevention equipment, to make sure they are working properly.
If a boat follows all the rules and gets checked by the grown-ups in the classification society, they get a special stamp of approval showing that they are safe and built the right way. That stamp shows that the boat can be trusted to keep people safe on the water, just like grown-ups make sure we're safe when we cross the street or use sharp things.