The deficit is a planned over spend by the Government. So when the Conservatives came to office they planned to spend £152Billion more than we were raising through tax. So they borrow that money which goes onto the National Debt.
While through austerity the Government are reducing their over spend, they are still overspending. So since 2010 the Government have spent £956 Billion more than we have, and that is added to the National Debt.
https://www.economicshelp.org/wp-content/uploads/2016/01/uk-net-borrowing.png
(Worth comparing on the map todays borrowing with Blair/Brown’s borrowing previously)
It is spent on the usual things, military, NHS, benefits/pensions