It’s someone who designs infrastructure, so roads, buildings, bridges, sewer systems, etc. Basically if you think about anything a construction worker builds, a civil engineer/team of engineers designed it. There are different categories of civil engineers. Structural engineers make sure a building can handle the weight of everything on/in it. Transportation engineers design highways. I’m a geotechnical engineer, so I deal with soils (dirt) and make sure the ground can support whatever is being built.