What is breaking inheritance?
When you change the permission levels for a child, you break inheritance. This means that it no longer inherits permissions from its parent. For example, you may only want a specific subset of users to access a certain subsite. In this case, you would break permissions inheritance and set a higher necessary level.
How do I break permission inheritance in SharePoint online?
Break permission inheritance in SharePoint
To break inheritance and assign unique permissions, follow these steps: Go to the list, library, or survey and open it. Go to the Permissions page using the steps in the previous section. To break permissions inheritance from the parent, select Stop Inheriting Permissions.