Let's suppose you put a neutrally floating object (an object with the same specific gravity as the fluid around it) into a continuously stratified fluid. If you lift the object up, it becomes relatively heavier than the fluid around it, and if you push it down, it becomes relatively lighter, and restitutive force acts on it to return it to its original position. The strength of this restitutive force changes with the vertical movement of the object, so if you assume the movement of the object does not disturb the fluid, and ignore the effects of viscosity, then the object would make a simple harmonic oscillation. The frequency (or period) of this oscillation is called the Brunt-Väisälä frequency (or period).
This frequency is proportional to the square root of the change in density on a vertical axis, so it is used as a parameter to express the strength of stratification in a fluid.