Users inherit permissions from groups

When working with Record Level Security it is important to keep in mind that users inherit permissions from the groups to which they belong. A simple illustration may suffice (see below for more details).

As the name suggests, all users are members of group Everyone. Let us give Everyone permission to Edit this record:

Now when we check the permissions of the Admin group we see that not only is the Display permission grayed out and uneditable, but so is Edit:

Note: If your objective is to remove permissions for a user / group and you find that a permission you wish to remove is grayed out and uneditable, a likely reason is that the user / group has inherited the permission from another group to which they belong which has been added to the Security box.