I've had a lot of trouble getting my head around the so called Irish border Brexit backstop and this article from the BBC goes some way to explaining it.
The problem here is that Northern Ireland (as part of the UK) has a physical land border with the Republic of Ireland, and as things stand at the moment, there is no border, with trade and people passing freely, without checks, as is the case right across the EU.
But Northern Ireland is a special case, because of the border and most people do not want to see a return of a hard line being drawn once again. The backstop is like a safety net, a kind of compromise, where NI would align with some EU rules and good entering the territory would be liable to inspection.
It seems to me that Brexit has and will create more problems than were ever foreseen by anyone who voted to leave.
Many now believe (hindsight is wonderful isn't it?) that the issue should now be put to the people again. A bit like loading a saved game in your favourite video game and picking up from where you were before the shit hit the fan.
This video also helps to explain this extremely complicated situation.