It’s about “standing.” Whether the religious right has the right to appeal on behalf of Prop 8. AP has a good write up:
A federal judge last year struck down Proposition 8, declaring that it violated the civil rights of gay and lesbian Californians. The coalition of religious and conservative groups that sponsored it appealed, but then-Gov. Arnold Schwarzenegger and then-Attorney General Jerry Brown, who both support marriage rights for same-sex couples, refused to do the same on the state’s behalf.
Expressing doubts about the ability of initiative proponents to wage an appeal on their own, the 9th U.S. Circuit Court of Appeals asked the state high court to weigh in before it tackled the broader constitutional issues.
So today’s expected decision is about whether the initiative proponents can appeal the case or whether it has to be the state of California.