Basically, they said that he lied to the Queen:

The supreme court has ruled that Boris Johnson’s advice to the Queen that parliament should be prorogued for five weeks at the height of the Brexit crisis was unlawful.

The unanimous judgment from 11 justices on the UK’s highest court followed an emergency three-day hearing last week that exposed fundamental legal differences over interpreting the country’s unwritten constitution.

………

Then, giving the court’s judgment on whether the decision to suspend parliament was legal, Hale said: “This court has … concluded that the prime minister’s advice to Her Majesty [ to suspend parliament] was unlawful, void and of no effect. This means that the order in council to which it led was also unlawful, void and of no effect should be quashed.

………

She [Hale] added: “The court is bound to conclude, therefore, that the decision to advise Her Majesty to prorogue parliament was unlawful because it had the effect of frustrating or preventing the ability of parliament to carry out its constitutional functions without reasonable justification.”

The judgment said: “This was not a normal prorogation in the run-up to a Queen’s speech. It prevented parliament from carrying out its constitutional role for five out of a possible eight weeks between the end of the summer recess and exit day on 31 October.

………

The court stopped short of declaring that the advice given by Johnson to the Queen was improper. It was a question. they said, they did not need to address since they had already found the effect of the prorogation was itself unlawful.

That denial at the end seems to me to be legalese for, “I see what you did there.”

I’m not sure how the politics will play out, but it seems likely that it’s better for people not named Boris Johnson than it is for people named Boris Johnson.