What is a network code on a mobile phone?

Excuse me, I'm trying to understand the concept of a network code on a mobile phone. Could you please elaborate on what it is? I'm aware that it's somehow related to connecting to a specific mobile network, but I'm not quite sure how it works or why it's important. Could you explain in simple terms, perhaps with an example or two? I'm really interested in learning more about this topic, so your help would be greatly appreciated.