La tronche is a colloquial word for “face”. It is a synonym of le visage (which is actually not very common in colloquial French) and la figure.
Les gosses des voisins jouaient au ballon et ils me l’ont envoyé en plein dans la tronche.
The neighbors’ kids were playing with a ball and they threw it right into my face.
J’aime pas sa tronche.
I don’t like his face.
It can also refer to the facial expression, especially when it shows a negative emotion.
Qu’est-ce qui t’arrive ? Pourquoi tu fais cette tronche ?
What’s going on? Why the sad face?
The expression faire la tronche (or, less colloquially, faire la tête) means “to sulk”, “to be mad at someone”.
Pourquoi tu me fais la tronche ? Qu’est-ce que je t’ai fait ?
Why are you mad at me? What did I do to you?
Listen to the examples: