Build process *must* not depend on the Internet access. You must be able
to preliminary download all necessary source code needed for the skel.
And as in most package build systems, that retrievable source code is
called "distfile".

There are no requirements how you will get it. But in most cases that is
either downloadable tarball or archive created from specific VCS commit.
Modern DVCS'es commit is already self integrity checked tree of files.
But tarball is just a bunch of bytes. You have to provide the URL where
to get it, and checksum(s) to verify against, to be sure that it is not
tampered or altered somehow. Moreover many distribution sites also
provide a detached cryptographic signature of the tarball, which you can
verify against some know author's public key.

There are various [Build/Distfiles/Fetchers] you can use.
Read the [Build/Distfiles/Tutorial] on how to prepare a distfile.
Downloads can be pre-downloaded and [Build/Distfiles/Cache]d.