Since the State of the Union is one of the few times of the year people pay attention to politics, you might want to brush up on your civics. For this I would recommend Ben’s Guide to the U.S. Government. Yes, it is for kids, but that makes it especially easy and quick to read.
So, for instance the entry on the State of the Union reads:
The State of the Union is an annual address presented before a joint session of Congress. It is required by Article II, Section 3, U.S. Constitution