A first assist is the "first" assistant to the surgeon who is operating.
In academic hospitals, it is usually a job assigned to a junior resident. In community practices, the assist may be a CSA, RNFA, PA, even another physician.
The amount of work done will vary from surgeon to surgeon, case to case and assist to assist. I have some whom I will let close the wound and others whom are only holding retractors (and some cannot even do that well).