Well… I learnt this in the first year of uni! It’s an area of fluid dynamics – basically how fluid moves – which is a very very complicated topic!
For a fluid such as water – i.e. one that isn’t thick and gloopy – Bernoulli’s principle says that any increase in speed of the fluid causes its ‘potential energy’ to decrease. This is the same as pushing rock over a cliff (you may have covered this in maths/physics). At the top of the cliff, the rock is not moving, but has the potential of moving very fast once you chuck it off the cliff. This is known as potential energy, and when you are taking about chucking rocks off cliffs, it depends on how high the cliff is.
So Bernoulli’s Principle is very simple, but can be applied to lots of different equations and complex fluid movements.