Google Keep opens a dialog that shows some information about your notes, some details about "upsync", "downsync", preloaded chunks and app init. If you enable "Show user text content", the debug page will also include the text of your notes.
I'm not sure if Google actually intended to show this debugging option in Google Keep's navigation menu. This looks like an internal debugging feature that's used by Google employees. Google Keep's Chrome app and Android app don't include this feature.
Update (a day later): It's gone.