1) A reference to the 10th Amendment to the U.S. Constitution
2) A code word that politicians use to signal to racists that they too, are racist. Used in the past to signal that you support segregation and Jim Crow laws.
When Strom Thurmond said he believed in State's Rights, he really meant that he wanted to keep segregation legal in the South.
If you believe in States' Rights but want to pass the Constitutional amendment banning gay marriage, you are a hypocrite.