Usually you're responsible for scheduling the residents for their shifts, scheduling lectures, and supervising the residents in an administrative role.
Basically, a chief resident is a supervisor of sort. They're usually in the process of being groomed for an academic position after they finish.
Some programs automatically make every senior resident a chief resident (i.e., surgery programs), but this is very rare in emergency medicine. In fact, I do not know of a single program that does this.
Most programs choose 2 or 3 senior residents to be their chiefs during their last year of residency. Some programs require an additional year when you are selected (and accept) as a chief. This is very common in many pediatric and medicine programs, but isn't that common in emergency medicine programs. Only a handful come to mind.