In a well-designed library, it does what it looks like - whatever "~" means in the language of that domain. In a badly-designed library it could do anything. Just like .add().

(It's normal for the same symbol to mean different things in different domains, even in Java. If a and b are BigIntegers, a.add(b) means one thing; if a and b are Wicket components, a.add(b) means something quite different. Wicket remains a respected, well-designed library even though it "overloads" the word "add" in this way)

(Another reply points out that it has the same meaning on numeric types - bitwise negation - as in Java, but I doubt that's what you're asking about)