A wolf-whistle is a particular loud whistle — two notes, one sliding up and one sliding down — that a man makes at a woman in the street to say he finds her body attractive. It is both a noun ('he gave her a wolf-whistle') and a verb ('he wolf-whistled at her'). The important thing is not the sound but what English speakers now feel about it. Fifty years ago it was often shown in films as cheeky or even flattering. Today it is widely treated as a form of street harassment: unwanted, aggressive, and something that makes women feel unsafe rather than admired. In some countries it can even be reported to the police. So when the word appears in modern English, it almost always carries disapproval — writers use it as an example of bad behaviour. Because of that history, you will also often meet it in discussions of how attitudes have changed ('back then, wolf-whistling was normal'). Note the spelling: 'wolf-whistle' with a hyphen is the usual form for both noun and verb, and 'wolf-whistling' is what you call the behaviour in general.