Fiscal Responsibility - A Translation Into Plain English

Everyone, it seems, is a fan of fiscal responsibility. After all, who on Earth would advocate fiscal irresponsibility? 

In March, John McDonnell set our Labour's stall for the next few years when he made a detailed speech about fiscal responsibility. Backed by some world renowned economic thinkers, it couldn't be further from the Tory economics of austerity. But what exactly has he signed Labour up to do, is it realistic, and how can we explain it to people who still think debt and deficit are the same thing?