Git is a distributed version-control system. Lilypond itself is developed using Git. It's useful to keep your .ly files in a Git repository: it ensures you can get back old versions of your files if you take a piece in the wrong direction, it makes it easy to share your work with others and incorporate each other's changes, and it lets you look back to see when you made particular changes. But if you're printing out your Lilypond parts, it's easy to get confused about which is the latest version, or what source a print-out was generated from.
The usual workaround is to use the date, but this only tells you when the PDF was generated, not when the source was written. This snippet is some trivial Scheme code that declares a \commit variable containing the current HEAD of the Git repository. (It's simple to change it to show the hash of the file, or the date of the HEAD commit, or the revision number from Subversion, CVS, or whatever else you use. It just runs a command and expects the output to be one line.)
Please ignore the displayed output of the snippet. It's there because all snippets need to produce output. The interesting stuff is in the file itself.