Largely means mostly or for the most part, but not completely. When something is largely true, it's true in most cases or in the biggest ways, even if there might be some exceptions.

If your science project was largely your own work, you did most of it yourself, though maybe a parent helped you cut some materials or a friend gave you one idea. If a book is largely about dinosaurs, dinosaurs are the main topic, even if a few chapters discuss other prehistoric creatures. When historians say that Rome fell largely because of internal problems, they mean internal problems were the biggest reason, though other factors played smaller roles.

The word helps you be accurate without exaggerating. Instead of saying “everyone” when you mean “almost everyone,” or “always” when you mean “usually,” you can say something is largely true. A team might win largely because of their excellent defense, acknowledging that offense mattered too. A student might succeed largely through hard work, recognizing that good teaching and natural ability also helped.

Largely is similar to mainly, mostly, or primarily, and it's useful when you want to identify the biggest factor while admitting others exist.