Quick Start - Nix Reference Manual says:
Install some packages from the channel:
nix-env --install --attr nixpkgs.hello
This should download pre-built packages; it should not build them
locally (if it does, something went wrong).
Why “This should download pre-built packages”?
Does that require
-b, which is absent in the example?
By default nix tries to fetch the pre-built packages from http://cache.nixos.org/. If there are none available it will build them from source. This flag will prevent this and just not install the package when there is no binary pre-built.
I hope that made it a bit clearer, feel free to ask more if you need more information.