Ok so in effect:
The UK voted to leave the EU approximately 2 years ago and the leadership of the country was replaced with someone in favour of staying in.
Since the vote, the prime minister has been debating with the EU for compromises they won't agree with and also refuses to simply leave with no deal --atleast until recently..
As of this moment, the 'deal' thats been agreed upon (atleast in theory) is a series of policies that ultimately keep the UK in the EU (following its regulations etc) so it's not really leaving and we also lose all power in the EU at the same time, and it's being debated as to whether we should have this.
Now the other party is offering to give people a new vote on exiting the EU and because the current party is likely to lose all confidence votes and the next election, the choice is pretty much between getting a deal now that keeps us in the EU but with less power or voting to remain..