Vortex
Vortex is a spinning flow of water or air that forms a funnel shape, like the swirl inside a flipped bottle.
Think of it this way
When you drain a bathtub, the water near the drain starts to spin. It forms a wide circle at the top and pulls down to a narrow point at the drain. That funnel shape is a vortex. Things near the edge spin slowly, but near the center they spin fast and get pulled down.
Explaining vortex by grade level
When you flip a bottle of water, the water can spin in a circle as it pours out. That spinning makes a hole in the middle shaped like a cone. The air goes up through that hole while the water swirls down. This spinning shape is a vortex.