A lot of very different things. A degree in computer science basically just means you work with computers. I was a CompSci major in college with a specialization in Networking, which means I mostly dealt with stuff regarding networks: running cables, setting up subnets, getting Mac to play nicely with literally any other kind of OS on a network, setting up switches, making wall jacks for Ethernet, etc etc. Programmers are also CompSci majors (typically). I think game development still falls under it, broadly? I know it did when I was in college. It's like asking "What do mathematicians do?" The answer is math, like how the compsci answer is "Computers"