From the SCOTUS Blog
This is big news. Anyone who gets a pardon from Trump can still be prosecuted on similar state charges.
Some commentators had thought that a court might take a knife to what's called the separate sovereign exemption where courts have ruled that state and federal governments are two distinct groups and that trying someone for similar crimes in both places would not violate the double jeopardy clause of the 5th Amendment to the US Constitution.
But the context here is that if the court ruled that this could be double jeopardy then you had the possibility of Trump issuing pardons that would cover federal law and this clearing you for being tried in state courts. But with the opinion of Justice Alito that will not happen.
Haven't read whole opinion but it was a 7-2 decision, with an unusual pair of Ginsburg and Gorsuch dissenting.
Hopefully others more well versed can offer an expert analysis.