The backstop is a sort of safety net or insurance policy meant to ensure no matter what happens with the rest of the Brexit negotiations, there won’t be a hard border between Northern Ireland and the Republic.
So, even if the rest of the UK leaves the EU with no trade or security arrangements, there won’t suddenly be border checks and restrictions on the island of Ireland.
At present, goods and services are traded between the two jurisdictions with few restrictions.
As the UK and Ireland are currently part of the EU single market and customs union, products do not need to be inspected for customs and standards, but after Brexit, all that could change.
If the backstop is invoked, it would mean goods coming into Northern Ireland from elsewhere in the UK would need to be checked to see if they meet EU standards.
This would also involve a temporary single customs territory, effectively keeping the whole of the UK in the EU customs union.