Locker service is a security layer automatically injected into your Lightning Component code by the platform to ensure that information the component can access stays within the scope of content on the page that only each component manages. This is important for users since they may configure a Lightning Page (via Lightning App Builder) with Lightning Components from multiple vendors and they need to be confident they are isolated from each other.

Salesforce provides the Locker Console as part of its Lightning Web Component Developers Guide website (https://developer.salesforce.com/docs/component-library/documentation/lwc). The following screenshot shows the output from the Benchmark button, which can be used to run performance tests on fragments of Lightning Web Component code:

Take the time to review the rest of the features, such as the Locker API Viewer, Playground, and the Component Reference, which provides an interactive way to try out the base Lightning Components provided by Salesforce.