Caller is the user on behalf of whom the incident is being reported. Two cases arise here.

Case 1:

The affected user themselves are creating the incident, in which case Caller is the current user creating the Incident. There is another field Opened by, which in case will be same as the Caller.

Case 2:

A user or Service Desk agent is creating the incident on behalf of another user. In this case, Caller is the actual user affected by the issue. Opened by is the user who is actually creating the Incident.

Hope this helps.

Thanks.