When ! is used in Java

If you look at the codes, say if (!s.add(a))!""I've seen things like equals (Ninput.getText() What do you mean by interpreting this? If ! is in front of you, it's my first time seeing it, so I'm asking you a question.

And if (!s.add(a))), then the add method is executed, and there are two things that happen as a whole, whether the condition is true or false?