What's inside /etc/apt/sources.list

And also quite possibly inside /etc/apt/sources.list.d/

Upfront, the /etc/apt/source.list is a configuration file for Linux's Advance Packaging Tool, that holds URLs and other information for remote repositories from where software packages and applications are installed.
Same goes with files inside /etc/apt/sources.list.d/.

Basically, when an installation command is executed, eg. apt-get install <package_name> then the Package Management System looks inside /etc/apt/sources.list and /etc/apt/sources.list.d/ for the remote repository link, and searches for the desired application to be downloaded.

" One of the reasons why there are two places for list instead of one, is in order to make sure data are not overwritten/lost during a system update. /etc/apt/source.list may be overwritten in update as this tend to happen with /etc/profile. That is also why custom configurations that generally goes inside /etc/profile file are preferably placed inside /etc/profile.d/ directory. "

A single link inside /etc/apt/sources.list file looks like this:
deb-src http://security.ubuntu.com/ubuntu eoan-security universe

On interpreting individual word:
deb-src

• debian source files that needs to be compiled. It can also be deb, which means precompiled debian source files.

http://securty.ubuntu.com/ubuntu

• The URL where the repository is stored

eoan-security

• The version name of the repository

universe

• How the software is available to end users
• It can be one of Main, Restricted, Universe or Multiverse

Overall:
deb (or deb-src) address distribution_name package_type_list