This is the fallback master page for uibuilder. If you are seeing it, you have an error in your configuration.
If you have just deleted dist/index.html from your local uibuilder folder, please restart Node-RED so that the src folder is used.
If you don't have a local src/index.html, please add one. The easiest way is to choose one of the built-in templates and click the "Load & Overwrite Files" button and accept the warning.
Please see the Documentation for details on how to use uibuilder.