“Porcelain” is the material from which toilets are usually made (and sometimes other fixtures such as washbasins). This is distinct from “plumbing” (the actual pipes and drains), where the porcelain provides a more user-friendly interface to the plumbing.
Git uses this terminology in analogy, to separate the low-level commands that users don’t usually need to use directly (the “plumbing”) from the more user-friendly high level commands (the “porcelain”).
PorcelainGit Porcelain
One of those programmer jokes…