Brunt-Väisälä frequency is a big and fancy name for something that happens in the ocean or in the air called "buoyancy."
When you go swimming, you might feel weightless and might float easily in the water. This is because your body is less dense than the water. The same thing happens in the ocean, where some water is heavier and some is lighter. When heavier water sinks to the bottom, it drags cooler water down with it. This process is called "mixing."
Now, the temperature and salinity of the water at different depths determine how much they weigh or how dense they are. The Brunt-Väisälä frequency tells us how fast the water moves and mixes depending on how much it weighs at different depths. It helps us understand how heat and salt travel in the ocean and how they affect the animals and plants that live there.
In the atmosphere, the same process happens with air. When warm air rises and cold air sinks, this creates wind and weather patterns. The Brunt-Väisälä frequency helps scientists understand how these patterns form and how they affect our planet.
So, remember: Brunt-Väisälä frequency is just a way to describe how water or air moves around in the ocean or the atmosphere depending on how heavy or light it is.