Guix supports transparent source/binary deployment, which means that it can either build things locally, or download pre-built items from a server, or both. We call these pre-built items substitutes—they are substitutes for local build results. In many cases, downloading a substitute is much faster than building things locally.
Substitutes can be anything resulting from a derivation build (see Derivations). Of course, in the common case, they are pre-built package binaries, but source tarballs, for instance, which also result from derivation builds, can be available as substitutes.