The accent is important because è and e are different words:
So the accent helps you distinguish them in writing.
It also reflects pronunciation: è is pronounced with an open e sound.
It is originally the past participle of rompere (to break), but here it is being used like an adjective.
So in this sentence, rotto describes the state of the bone: it is in a broken condition.
This is very common in Italian. Many past participles can function as adjectives:
Rotto is perfectly understandable and natural in everyday language.
However, for a bone, Italians may also use fratturato, which is more specific or medical:
So rotto is fine, but fratturato is more precise in a medical context.
This is a useful question because osso has an irregular plural pattern.
You may see:
Very roughly:
For many learners, the most useful form to recognize first is:
So:
This is a slightly advanced point, but it is worth knowing because osso does not behave like a completely regular noun.