Somewhere up in the code there are a few objects (hard to tell what they are from this code) that are named tryAgain, one, two, and three. Something is triggering an event on these that is calling actionPerformed.
I would "guess" that guess is defined as an int somewhere else. It looks like if the event came from tryAgain then you reset the guess. Otherwise you do the math as shown.
Does that make sense?
Greetings.
Albert