It means we pay no customs duty on goods coming or going to the 27 member states and trade on the eu terms.
Which lets face it the eu terms are going to be way better than any terms we can negotiate ourselves, because the deals negotiated we are one of 28. Who the fuck is going to give us better terms to supply only us, v supply us and 27 other counties. It's like the discount doe a small customer v a big one,
It also means no need for a back stop in Northern Ireland, because goods can flow freely.