An Office Manager, in the website platform, would typically be considered the 'broker' of a specific office location. With these privileges, the account gains the ability to view and edit any lead that is assigned to their office. (This includes leads who are assigned to other agents.) Additional functionality for office managers includes the ability to edit the profiles for agents in their office, create new action plans, and to setup and send campaigns and newsletters to lists that agents in their offices have created. In the case where there is an office site, the Office Manager has access to the 'Content' tab. 
Another use-case for creating someone as an office manager is for 'team lead's. In a brokerage, oftentimes there is a top producer or team lead who has agents who work underneath their book of business. In this case, each team member would have their own real estate license, but works leads based on what team lead assigns out. If a team lead needs access to the leads in their team, you can Create an Office Location with their team name then make them the Office Manager for that 'office location'.
To set up someone as an office manager simply...

1. Go to Office Tools > Offices.

2. Click on the Edit pencil for the office location of which you'd like to make your team member the manager.

3. Scroll down to the Managers Section.

4. On the right side of the screen use the drop down to designate an Office Manager.

5. Please be aware, at this time, Office Manager accounts cannot be deleted, so if you "Add New Manager", but then need to revoke that later, you'll simply need to designate someone else as the office manager in that drop down.

Note: The same account can be the primary manager and manager.

Please see this help article on the different user roles for more information, as office managers often just need to be Admins on the site.