flakes, in the most basic terms, is a solution to "well it works on my machine".

So say you develop something within a flake, you can then take that flake and reproduce it 100% on whatever machine you choose. so what ever your building in it if it worked on your machine, it's going to work on all machines.

It's also good for configuring your NixOS. your configs are within the flake and if something happens to your system or you want to replicate your system completely on another machine you can. i.e. you push your nixos config to a git repo and then you can clone it where ever and now you have your complete OS setup somewhere else. Like say if your system borks and you need to reinstall Arch or Ubuntu or whatever it will take awhile to get it set up completely has you had it before. With NixOS and Flakes it's like 10minutes tops to get back to where you were.

TL;DR lockfile with inputs (deps like nixpkgs) and consumable outputs (packages, shells, system)