It doesn't mean nothing. It means leaving the EU.
It has also been regularly qualified by the key Leave players, including May herself.
“Brexit does not mean partial membership of the European Union, associate membership of the European Union, or anything that leaves us half-in, half-out. We do not seek to adopt a model already enjoyed by other countries. We do not seek to hold on to bits of membership as we leave.
“The United Kingdom is leaving the European Union."
Theresa May, January 2017