In order to understand how the communication processes work in a computer network, it is helpful to know what they are based on.
While humans have a grammar and certain social rules in addition to their language, computers do not have that first. No one is prevented from building a machine that works on its own but is unable to communicate with other computers. Precisely because it does not master the standardized agreements that are made so that different hardware can communicate with each other. Just as a Russian can communicate with a Canadian if both speak a standard language - English.
For this purpose, different protocols have been developed over the years for different purposes. Someone who reads this website on his browser can do this thanks to HTTP, a protocol that governs the transmission and presentation of hypertext. On the other hand, with SMTP, no web pages can be presented, but electronic mail can be sent, while POP3 or IMAP, in turn, manage the way in which the mail is received.
Since these are still abstract, logical operations performed by other abstract constructs, such as programs, and the latter, in turn, involves the underlying hardware, an overarching rule comes into play. These are the ISO / OSI reference model from 1984 and from the 1970s the TCP / IP reference model. Both models describe at which level (software, hardware, network devices, internet) which protocol is used.