It's only an opinion, but it is a pretty important one.
The question asked of the European Court of Justice was a straightforward one: can the UK change its mind and withdraw its Article 50 notification any time it likes, and without conditions?
The European Commission's view was that changing your mind is fine, but that the other 27 have to agree unanimously to let you stay in. The British government simply said "we don’t want to change our minds, and so you have no business even deciding this case".
Well, on Tuesday the advocate general of the court gave his opinion; it's not a binding opinion, the judges can overrule him and decide something else, but for the most part (80% of the time or more) they don't.
He said yes, Britain can unilaterally revoke Article 50 right up until the moment the Withdrawal Agreement is signed off, and there's nothing the EU 27 can do about it. There were a couple of conditions: the revocation has to be done through proper Parliamentary processes, and there can't be any gaming the system. The British government has to mean it, not just be a negotiating tactic to win more time for negotiations.
His reasoning was that to do anything else would be to effectively expel a member country that had changed its mind about leaving, and that would be both unfair and un-European. I'm not a lawyer, but having listened to the original hearing in Luxembourg, I would be most surprised if the judges came to a different conclusion.
So why does any of this matter? After all there's no immediate prospect of a British government deciding it wants to stay in the EU. It's about whether - if a second referendum for example said we should Remain - the EU would be able to change the terms of our membership.
Many have argued (indeed Michael Gove did so again this week) that there was no point in trying to stay in at this stage, because all our exemptions and opt-outs would be lost. The budget rebate, staying out of Schengen and the Euro, they'd all be gone, because the EU would demand they be dropped before it let us revoke Article 50.
Well whether or not the EU27 would ever have been tempted to try and do this (there are a lot of 'ifs' in all this), they now can't.
So to summarise:
IF the judges agree with the advocate general and
IF the Withdrawal Agreement doesn’t get through Parliament and
IF the Government then decides the only way out of the Parliamentary stalemate is to hold a 2nd referendum and
IF there was to be an option to Remain on the ballot paper and
IF Remain won
THEN: we could stay, on the same terms as we have today.