I'm hoping this is the right category, since it's set theory. If it's not I'll move it if someone knows a better place for these questions

Prove that A X (B_{1}U B_{2}) = (A X B_{1}) U (A X B_{2})

I am actually stuck on the B_{1}B_{2}in this problem. It's some notation aspect that is keeping me from understanding what the problem is asking for. Are B_{1}and B_{2}separate sets?