Nixpkgs (pronounced "Nix packages") is a big collection of Nix expressions, packages (who'd have thought...), and build utilities for the Nix ecosystem. It is sometimes also called a "Nix standard library" (or Nix stdlib).
Nixpkgs is a mono repo, containing over 80,000 package definitions and the utilities to maintain this set of packages. It also contains NixOS modules and anything required to build and deploy a full Nix system. Many tools in the Nix ecosystem depend on this standard library to function.