One is California's same-sex marriage ban Prop 8 that was struck down by a federal judge. If the Supreme Court upholds that decision, same-sex marriage could begin again in California. Justices could also use the case to legalize same sex marriage across the country.
The other decision is on a challenge to the federal Defense of Marriage Act. It's a federal law that defines marriage as being only between a man and a woman. The law prevents married gay couples from receiving a number of federal benefits.