Main Agents could be the main contact at an agency, such as an Administrator, or the Manager or owner of an agency. Every organisation can have a Main Agent. You can mark any Person as the Main Agent by checking the box on their record:

They are highlighted on the list of agents by an asterisk at the end of the name.

When creating a Campaign, if you only want to contact one person at each Agency, you can Filter your Agents results to include your Main Agent only.