What does E.T. stand for?

Q: What does E.T. stand for?
A: The national anthem.

This is a pun. The joke is that stand for can mean both "stand up on his feet for", and "mean".

In this joke, there is confusion on the part of the listener, because E.T. can be a general noun, and also a specific one. E.T. could be an abbreviation meaning Extra Terrestrial, or it could be a movie character from the movie E.T. the extra terrestrial.

I could imagine another version of of the joke with an answer like "The rights of oppressed people everywhere", because "stand for" can also mean "support a cause"