This morning, the EU has agreed to extend Brexit until 31 January 2020.
By passing the Benn/Surrender Act against the Prime Minister's wishes, opposition MPs forced his hand. They dragged him through the courts to force him to seek and then accept an extension. That's why it's known as the "Surrender Act" - it gives up our power over the Brexit deadline and gives that power to Brussels to decide.
Brussels has decided January 31 is the new deadline. The PM wanted to pass his deal and leave this Thursday but Parliament said no. They've blocked "no deal" and they've blocked a deal. It can't carry on like this. That's why Boris will call for an election today, to break this deadlock. If he wins a majority he'll pass his deal and we don't have to wait until January, we could do it straight away.