What is the relationship between velocity and pressure?
I'm fairly certain that you are referring to the pressure of fluids. Scientifically fluids include both liquids and gasses, because fluids would be anything that doesn't have a definite shape.
Bernoulli's principle defines a relationship between the fluid pressure and the speed (velocity) of that fluid. It is an inverse relationship, which means that as one thing increases the other decreases. For fluids it works like this: As the velocity of a moving fluid increases, the pressure that it exerts decreases. I've always thought that just sounds weird. Something moving faster has less pressure. But it's true and explains how all kinds of stuff works.
Most teachers use wings and flight in conjunction with Bernoulli's principle. Simplest explanation for that is that a wing is flat on the bottom and curved on top. As it moves through the air, the air going over the top of the wing moves faster in order to cover the longer distance in the same amount of time. That creates a lower pressure above the wing and a higher pressure below the wing. Fluid pressures want to be equal, so the high pressure attempts to move toward the low pressure. Because the wing is in the way, the wing (and airplane) gets lifted up. Bernoulli's principle explains other things too. It's why baseballs curve, soccer balls bend, Frisbees fly, and shower curtains get pulled in.