Turning this feature on enables live code editing in Visual Studio Online. After this feature is enabled, a link to the editor ...

Turning this feature on enables live code editing in Visual Studio Online. After this feature is enabled, a link to the editor will appear in the quick glance section of the DASHBOARD tab. 

If 'deployment from source control' is enabled, this can overwrite any changes made in the editor. We recommend that you do not enable 'deployment from source control' if you want to edit the site contents directly in the online editor.