By default, all sub-sites, lists, and libraries within a site inherit permissions settings from whatever is directly above them in the site hierarchy.

For example, a Library item inherits permissions from the Library that it is in. A Library inherits permissions from the site is on. In each of these cases, the content from which permissions are inherited is called the parent.

In some cases, your site might contain content that’s only meant for certain users or groups. 

For example, if you have a Library on your site that contains sensitive data, you might want to restrict who can see it. To do this, you break permissions inheritance, and then edit permissions for the content on its own permissions page.

Break inheritance from the parent:

1. Open the Library that you want to restrict access to.
2. On the Library Tools tab, click Library to open the gallery of commands specific to the Library.
3. On the ribbon, click the Library Permissions button.
4. Click the Stop Inheriting Permissions button.

Here are two important things to note:
- The Library still has the same permission settings that it did before. But now, instead of inheriting permissions from the parent, it has its own copy of the parent’s permissions. 

- Changes that you make to the permissions settings for the parent site will not be inherited by this Library.

Remove groups or users you don’t want:

1. In the Name section of the permissions page, select the checkboxes for the groups or users who should not have access to this Library.
2. Click Remove User Permissions.

Grant access to groups or individuals:

1. On the permissions page for the Library, on the Edit tab, click the Grant Permissions button.
2. Type the name of the group or the individual you want to grant access to in the Users/Groups box.
3. Choose the level of permissions you want the group or individuals to have.
4. Click OK.

Reconfigure a Library to inherit permissions:

After you’ve broken permissions inheritance between a site, folder, Library, library, Library item, or document and its parent, you can restore inheritance at any time. 
1. Open the Library that you want to restore inheritance for.
2. On the Library Tools tab, in the Library gallery, click the Library Permissions button.
3. On the Library permission page, click Inherit Permissions.