So, what is it?

A customs union is a form of trade agreement between two or more countries. The EU’s customs union is made up of all 28 EU members plus Monaco

Countries in the union apply the same tariffs on imports from outside the union. So the duty on a television made in South Korea will be the same whether it arrives in Britain, Germany or France

If Britain stayed in the EU customs union, it would not be able to negotiate its own trade deals elsewhere in the world, because it would be tied to the tariffs set by the EU