SSRS Report Manager supports Windows authentication by default. There are few other authentication types supported, you can configure. See https://bit.ly/2LYNyRy. If you would like to use another type of authentication you should implement it through Reporting Services extension https://bit.ly/2yBJjmj.
When you would like to manage user access to Reporting Services it is realized by roles you set to its login. There are two areas of roles you set to users.
One is set in Report Manager site settings and its scope is System access and the second one on Report Item level (Reports, Folders, etc.)
In site settings you set roles for users that should have rights to manage Schedules for example.
Below is list of Report Item roles taken directly from MSDN. Roles are inherited to item by default from its container.
Content Manager Role | Item | May manage content in the Report Server. This includes folders, reports, and resources. |
Publisher Role | Item | May publish reports and linked reports to the Report Server. |
Browser Role | Item | May view folders, reports, and subscribe to reports. |
Report Builder Role | Item | May view report definitions. |
My Reports Role | Item | May publish reports and linked reports; manage folders, reports, and resources in a users My Reports folder. |
System Administrator Role | System | View and modify system role assignments, system role definitions, system properties, and shared schedules, in addition to create role definitions, and manage jobs in Management Studio. |
System User Role | System | View system properties, shared schedules, and allow use of Report Builder or other clients that execute report definitions. |
Report items, Reports, Path go to Report properties by click on right arrow next to the report, report item or folder name.

Go to Security item from the menu.

In case of folder you can go to Security through Folder Settings from main menu.

In Security section you can see users with accessed roles. You can Edit current security settings or add new user by clicking on New Role Assignment.

When adding new user add domain\user login and check roles you would like to access to the user.

When setting System roles go to site settings on the top menu of Report Manger.

Picture 7 – Administration roles