Every app user is assigned a profile that defines what they can see and do in the app user's UI: what menus, modules, records, and fields are visible, and what actions they can perform on those records. When you preview or publish an app, profiles are defined by records in the Profiles module:
The Profiles module is defined in the app developer's UI, but you create profiles in the app user's UI. The app's administrator is assigned a default Administrator profile that gives access to all records and actions. For normal (non-admin) users, you should create profiles so they can see and do only what's necessary for their roles.
The profile for an app user is given by the value of the Profiles lookup field in the user's Employees record. As this is a single lookup field type, a user can have only one profile at a time, but you can create as many profiles as you need for each app user role. You can also adapt the Profiles module to suit your organization's needs by adding or renaming fields and sections to the module in the app developer's UI.