The source mode is shown when clicking the HTML icon in the toolbar and lets you see/change the raw HTML source for the content.
Enable source mode
To enable it, make sure "Allow Source Mode" is enabled in the Source section of the configurator, and add
html to your toolbar buttons under Toolbar where you want the button to be available.
If desired, you can enable CodeMirror syntax highlighting for easier use of the source. Under the "Source" section of the configurator, make sure "Enable CodeMirror" is enabled.