Permissions, in CMS Made Simple, are used for two major and related purposes: enforcing security via access control, and interface simplification. Access control is mostly self-explanatory: you only allow users to take actions that they are authorized to perform. Interface simplification is the related process of only displaying interface elements to users who will need to use them. If someone performs the role of an editor, there's no point in showing them interfaces for updating templates.
CMS Made Simple has a built-in group-based permission system for implementing both purposes. This recipe demonstrates how to use it.