This is one of those big changes in the country of nix that is on the horizon. The flakes aim to improve certain aspects of nix: From Dolstra (RFC 49):
Abstract: This RFC proposes a mechanism to package Nix expressions into composable entities called "flakes". Flakes allow hermetic, reproducible evaluation of multi-repository Nix projects; impose a discoverable, standard structure on Nix projects; and replace previous mechanisms such as Nix channels and the Nix search path.
A series of developer posts describing flakes can be found here: TweagIO Flakes.
Notes from Alexander Bentyev on getting started through his NixCon.
Everything is a flake... nixpkgs, Nix, that serokell presentation, if there is a flake.nix in the root directory it is a flake.