What exactly does this mean? Could I do work with them? If not, why are they listed there? I am confused!
Retired (or emeritus) faculty are generally not available as research advisors for graduate students, although depending on the university's and department's policies, a retired faculty member may be able to hire postdocs and other technical staff ("soft-money" positions that can be eliminated if the funding isn't there).
As for why they're listed: in many cases, retired faculty still retain some active affiliation and involvements with their departments. For instance, they may be involved advising undergraduate students, teaching classes, or continuing to do their own research, but absent a group to work with. (As an undergraduate, there was one chemistry emeritus professor who was still working in the laboratory—at age 80!)
A retired faculty member ("professor emeritus") is, most importantly, retired. That means that they are no longer drawing a regular salary, and conversely, they do not have the regular obligations (teaching, research, working with students, serving on committees, etc.) that are part of a faculty job. However, the still have access to some departmental resources, and they typically choose to continue to do some kind of work. Some retired faculty still teach; this can mean effectively working as an adjunct in their former department. Others continue to do research, although how easy this is depends a lot on what kind of research they do. In the experimental sciences, it is usually too much labor to maintain an entire lab and oversee students working in it after retirement. On the other hand, people whose research does not involve large numbers of students and extensive facilities often continue to do research (albeit at a reduced pace). When I was a graduate student, one of my research papers was done under the supervision of a recently retired professor.
The upshot is that it is impossible to know whether a retired faculty member will be willing to supervise a graduate student. If you are interested in laboratory research, it's unlikely that an emeritus is going to have a fully functioning lab. Even if you are interested in more theoretical research, you should be aware that emeritus professors typically tail off their involvement with the department over time. Somebody who might still be on campus most days two weeks after official retirement might be a ghost a couple years later.