The backstop keeps the UK in the EU customs union and parts of the single market, tho I don’t understand exactly which bit(s)
It only comes into gorge if by the end of the transition period in 2021, during which the EU and UK are supposed to be negotiating a trade deal, no solution has been found to checking customs and trading standards of goods entering the EU from OUTSIDE (actually)
Originally it was going to involve a customs border between Ireland and GB, but the government's confidence and supply partners the DUP kicked up a fuss.