You can turn off inheritance, by clearing Inherit from parent. The list of access rights becomes available and you can edit it and set up different access rights. Any child objects of the item inherit the new access rights, unless you edit their properties to turn off inheritance.
In the following picture, a hierarchy of items has a common ancestor A. The Inherit from parent field shows that all the items have inheritance turned on except A and D.
The orange items (B, C, E, and H) inherit their access rights from A. The blue items (F and G) inherit their access rights from D.
If you change the access rights for A, the change automatically applies to the other orange items (B, C, E, and H). The change ripples down through the tree to all items that inherit their access rights from A. Similarly, if you change the access rights for D, the change automatically applies to the other blue items (F and G).
You are managing a car project and have created an Engineering group, which is composed of John, Sue, Mark, and Jane. You want everyone in the Engineering group to have full access to all the data in the car project:
You set up the access rights for the car project, giving the Engineering group full access to the project. When you create data in the project, it inherits its access rights from the project, so the Engineering group has full access to all the data in the project.
But Sue and Mark are writing the Design module, so you want to give them full access to that one module. Turn inheritance off for the Design module, and add an access entry for Sue that gives her full access to the module, and an access entry for Mark that gives him full access to the module.