Why do I need to know chut?

Because if someone tells you to chut, you want to avoid asking what it means. 

What does it mean?

Chut is the French equivalent of 'hush'.

The French online dictionary l'Internaute defines it as an 'invitation to be quiet', and another dictionary says it is “generally accompanied with a gesture of the index finger on the mouth.”

In other words, chut is the French version of the universal interjection 'shhh'.

If you spend time with French parents, you will surely hear them exclaim chut alors ! (be quiet for god's sake!) to their children. Do not mistake this for zut alors, which means 'darn it'.

Use it like this

Chut, le film commence. – Shh, the movie is starting.

Mais chut ! J'entends rien de ce qu'il dit ! – But hush! I can't hear a thing he's saying!

On lui a fait chut plusieurs fois de suite, mais il ne voulait pas se taire. – We told him to be quiet several times, but he didn't listen.

Synonyms

Silence – quiet

Tais-toi – Be quiet

Taisez-vous – Be quiet (plural)

Boucle-la – shut it (colloquial)

Ta gueule – shut up (vulgar)