In simple terms....in very very simple terms......
For years, blood has flown in the Island of Ireland concerning a border between Southern ireland and Northern Ireland and thanks to the Good Friday agreement, there has been an open border.......like driving into Scotland ....no visas, no passport control....nothing.
Everybody happy.......very happy.
NOW! Brexit is voted for by the Brits and when it comes to fruition, Southern Ireland will be in the EU but Northern Ireland wont......so after all these years....we will be back to square one....visas, passports etc to cross the border. The backstop has been conjured up so that this barrier between N I can be removed at a future date to restore free flow between the North and the South of the island.
That's my take on it anyway.