Okay kiddo, have you ever played in a pool or in the bathtub and watched how the water moves when you splash around? Well, when you play in the water like that, you are creating something called a free surface.
A free surface is a surface that is not fixed or attached to anything, and it can move around by itself. Basically, it's the surface of a liquid (like water) that doesn't have anything stopping it from moving.
You can think of it like the top of a drink in a cup. When you move the cup around, the drink inside moves with it, but the top of the drink (the free surface) stays level and smooth.
Free surfaces are really important in things like boats and airplanes because they can affect the way the object moves. For example, if a boat is not balanced properly, the free surface of the water it's floating on can cause it to tip over!
So, a free surface is a surface of a liquid that is not attached to anything and can move around on its own. It's important to be aware of free surfaces so that we can make sure things stay balanced and don't tip over!