Office is javascript API for any Office 365 products. More info can be found here https://docs.microsoft.com/en-us/office/dev/add-ins/develop/understanding-the-javascript-api-for-office#initialize-with-officeonready and function OnReady means when the API is successfully loaded by any Office 365 clients then this function is called.
And on this event I would like tell quasar do create Vue root app and render it. If my explanation is not enough do not hesitate to ask me more questions and I would try to explain in more details.
Thanks you
Jan Vaca