Locker Service brings multi-tenancy to the browsers. This is done by sandboxing of code and isolation of elements, thereby setting them apart from the rest of the system. Locker acts as a Virtual iframe that helps to bring all the security benefits - minus the drawbacks of UI for an iframe.
Lightning Locker is a powerful security architecture for Lightning components. Lightning Locker enhances security by isolating Lightning components that belong to one namespace from components in a different namespace
Locker source is open-source Javascript files maintained by Salesforce. When Single Page Applications (SPAs) are built using Lightning components, then Security can be enforced by LockerServices.
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