Not really sure why it is called a backstop but it is what is making the Brexit negotiations so difficult although if you ask most people, they don't really know what it is so let me explain.
Currently Northern Ireland, as part of the UK, and the Republic are both in the EU and trade moves between the two countries effortlessly with no customs checks but after Brexit, if there is no deal, both sides will be in different Custom Unions which could mean Customs checks at the border of the two, a border which is currently just a sign which says 'Welcome to Northern Ireland'.
A no deal Brexit would mean a hard border be erected between the two as all goods from a Non-EU country entering the EU zone needs to be checked, so a place for goods to be stopped and lorries and cars will need to be installed and anyone who knows the recent history of Ireland and the troubles, can see why that is to be avoided.
The EU has proposed what is known as a 'backstop' which includes Northern Ireland alone remaining in the EU's single market and customs union but the ruling Democratic Unionist Party in Northern Ireland objected to this as it would be seen as treating Northern Ireland treated differently and would threaten the union with the rest of Britain.
The Conservative Government, under Theresa May, agreed to whole of the UK staying in the EU Customs Union for as long as it took until another arrangement could be put in place but many Conservative MPs at Westminster voted it down, three times actually, as they were concerned it would leave Britain permanently trapped in the EU Customs Union and if Britain is part of the EU Customs areas, it cannot make deals with other nations.
Theresa May resigned over it and new Prime Minister, Boris Johnson, says he is committed to getting rid of the backstop, and apart from building a bridge between Northern Ireland and Scotland, he has not yet come up with a viable solution.
The Backstop then is how to deal with the trading of goods between Ireland, an EU Member, and Northern Ireland which will be outside of the EU and the thing which could scupper Brexit altogether.