Hydraulics is the science of the way fluids move. It's the branch of physics and engineering that focuses on the mechanics of liquids. If you want to be a civil engineer, you should probably learn about hydraulics.
If you decide to study hydraulics, you'll learn about how fluids like water and oil are used in mechanical tasks. You might read up on hydraulic jacks, which can lift cars using oil that's forced into a cylinder using a kind of pump; or hydraulic fracturing, also known as "fracking," which uses a liquid under pressure to fracture rock and extract oil or gas. Hydraulics comes from the Greek hydraulikos, or "water organ," an ancient musical instrument.