What is 'Just My Code'?

Visual Studio has an option (under Debugging / General) "Enable Just My Code"

What is 'Just My Code'? Visual Studio doesn't explain the feature.

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How do I enable debugging in Visual Studio?

In the Visual Studio toolbar, make sure the configuration is set to Debug. To start debugging, select the profile name in the toolbar, such as <project profile name>, IIS Express, or <IIS profile name> in the toolbar, select Start Debugging from the Debug menu, or press F5.

From Visual Studio Docs:

Enable Just My Code: The debugger displays and steps into user code ("My Code") only, ignoring system code and other code that is optimized or that does not have debugging symbols.