You know when you're trying to sort something out in a big group of people, and it finally gets sorted... and then someone says one thing and the whole discussion starts again?
That's probably how Theresa May felt about Chequers (then again, she really should have known it was coming).
What it means: The way it works is this: cabinet ministers come up with a set of proposals, they get written up into what's called a 'White Paper', MPs propose changes (called 'amendments') to the Paper, they vote on it, and the end result gets sent to the EU.
Yesterday was the 'voting on amendments' part, and things got tense. Basically, a group of 'Hard Brexiteer' MPs – the ones who want as little to do with the EU as possible – proposed an amendment that HMRC should not be able to collect any kind of European taxes (including VAT) unless the EU is doing the same for the UK... which is highly unlikely to happen. The amendment passed by 305 to 302 votes.
The reason this is a big deal is because the way the White Paper previously approached this issue of the 'customs union', or the arrangements on what does and doesn't get taxed across borders, was seen as one of the few parts of it that the EU might just roll with. But this change is probably not going to gain their approval.