Okay kiddo, have you ever been on a boat or did you ever see one on the water? Sometimes, the water where the boat is can be really dangerous and it can make the boat tipsy or even sink. So, to keep people safe, the people who know a lot about boats and water have something called a "small craft advisory".
This is when they tell the people who want to go out on their boats that the weather is too dangerous, too windy, or too wavy for small boats. When they say this, they are telling them that it's not safe to take their boats out on the water, and they should wait until the weather is better.
Think of it like your parents telling you that you can't go outside to play if it's raining really hard or if there's lightning. Safety is the most important thing, and when the small craft advisory is in place, it means that the water is not safe for small boats. So, it's best to listen to the advisory and stay safe on land until the weather gets better.