LockerService

The LockerService architectural layer enhances security by isolating individual Lightning components in their own containers and enforcing coding best practices.

• LockerService implicitly enables JavaScript ES5 strict mode.
• A component can only traverse the DOM and access elements created by a component in the same namespace. You can’t use component.find("button1").getElement() to access the DOM element created by <lightning:button>.
• LockerService applies restrictions to global references. LockerService provides secure versions of non-intrinsic objects, such as window. For example, the secure version of window is SecureWindow.

Salesforce have decoupled the Content Security Policy (CSP) from LockerService and have made it available as a critical security update.

References:
LockerService API Viewer app: http://documentation.auraframework.org/lockerApiTest/index.app?aura.mode=DEV