what is it called when a supervisor fills in for a subordinate. I as a supervisor was told by a subordinate that I had to work her position while she was out on Friday.I called my manager and discussed the issue but was told by him that it would be looked into. should I, or is it right by law that I do so?
It's not unusual for a supervisor to provide cover for a subordinate when they are unavailable (sick, holiday etc) - but it's rather unusual for a subordinate to tell a supervisor to do this. Typically a supervisor would be responsible for making sure the role/work is covered and it's up to them to work out how this happens - whether the work gets delegated to another subordinate or the supervisor does it themselves.
A good leader is one who is willing to step in and cover for a subordinate. I'm going to assume you're new to this company, and perhaps the subordinate was just telling you how past supervisors would have done it? As the leader of an organization I've scrubbed toilets if they needed cleaning. It's what a good leader does. If the employee being gone prevents the job from getting done, then you fill in for them.
If I were you, I'd talk it over with the manager. Sounds like a policy needs to be implemented to deal with how to cover for absences. Maybe a policy requiring employees to attempt to find coverage is needed. Maybe your job should be to find someone else to cover it in advance. Maybe you need more staff to cover for absenteeism.