CAN someone please explain to a rather aged reader what is the ‘deal’ that the Prime Minister wishes Parliament to agree to prior to this country leaving the European Union?

I can understand the position of those who wish this country to remain a member of the EU trading freely inside a ‘common market’ and accepting the obligations that go with membership. I can understand the position those who wish to leave- full stop! I can also understand the view of those who wish to be part of a customs union. But what I cannot understand is exactly what is Mrs May’s deal ?

Can someone please explain?