The straight answer is a modem bridges and translates between layers in the OSI model, typically layers 1&2 (physical and data) on one side (your coax for cable or fiber) and layers 3&4 (IP and TCP, the typical protocols for home networking) on the other side. We used to call modems the devices that would do that translation over a phone line, and we would call the devices that connected two different network types bridges (like coax or microwaves to Ethernet or token ring), but now we call what are really bridges modems for home networking deployments.
To get between ELI5 and PhD, dig into some good telecom textbooks and focus on the fundamentals like the OSI model, time-division multiplexing (TDM), frequency-division multiplexing (FDM), DOCSIS, how to send binary data over radio, etc.
The straight answer is a modem bridges and translates between layers in the OSI model, typically layers 1&2 (physical and data) on one side (your coax for cable or fiber) and layers 3&4 (IP and TCP, the typical protocols for home networking) on the other side. We used to call modems the devices that would do that translation over a phone line, and we would call the devices that connected two different network types bridges (like coax or microwaves to Ethernet or token ring), but now we call what are really bridges modems for home networking deployments.
To get between ELI5 and PhD, dig into some good telecom textbooks and focus on the fundamentals like the OSI model, time-division multiplexing (TDM), frequency-division multiplexing (FDM), DOCSIS, how to send binary data over radio, etc.