The phrase "not guilty" is the opposite of "guilty". This is what an jury announces if they decide that someone hasn't committed a crime.
There's another word, "innocent", which is also the opposite of "guilty". However, there's a difference between "not guilty" and "innocent". If you're "innocent", it means that you didn't do anything wrong. So someone who's been accused of a crime will say:
I'm innocent! I swear! I didn't kill him!
But "not guilty" is a "verdict", which means that it's an official decision which a jury makes.
Another way to say that the jury has found someone "not guilty" is to say that the jury "acquitted" the defendant:
They acquitted her. Can you believe it?