What is network mask code?

Could you elaborate on the concept of network mask code? I've heard it mentioned in discussions related to networking and IP addresses, but I'm not entirely clear on its significance and purpose. Could you explain in simple terms what a network mask code is, and how it is used to define network segments or subnets? Additionally, I'd like to understand how it differs from other network addressing concepts, such as IP addresses and gateways. Your insights would be greatly appreciated in helping me gain a better understanding of this fundamental aspect of networking.