What are Salesforce Lightning Extensions?

Salesforce Lightning Inspector Chrome Extension
The Salesforce Lightning Inspector is a Google Chrome DevTools extension that enables you to navigate the component tree, inspect component attributes, and profile component performance. The extension also helps you to understand the sequence of event firing and handling.
The extension helps you to:

Navigate the component tree in your app, inspect components and their associated DOM elements.
Identify performance bottlenecks by looking at a graph of component creation time.
Debug server interactions faster by monitoring and modifying responses.
Test the fault tolerance of your app by simulating error conditions or dropped action responses.
Track the sequence of event firing and handling for one or more actions.