The backstop is something of an insurance policy against a return to border checks (a "hard border") between the Republic and Northern Ireland.
Although the backstop would be enacted as a last resort if no trade deal is reached between the EU and the U.K. in a 21-month transition period after Brexit and is designed to prevent the return of a "hard" border between Northern Ireland and the Republic. It's contentious because it would mean that Northern Ireland (and potentially the rest of the U.K.) remains in a customs union with the EU for an indefinite amount of time.
The backstop could not be ended without the EU's permission. Dutch Prime Minister Rutte told CNBC that a collective agreement was needed that if the backstop was needed at all, "it will be as short as possible."
"Nobody on the EU side will want to keep the U.K. any longer in the backstop than necessary; only 'til a new arrangement for that border has been found. That level of confidence, that level of trust has to be on the table," he said.
EU leaders have refused to renegotiate the Brexit deal or backstop, however, only offering assurances that it is a last resort and unlikely to be used. The EU has previously said it is willing to consider an extension to the U.K.'s departure date.