There are some nice answers on this question. I’ll try to add a more broad answer, namely about what these kinds of lines/headers/trailers are about in current practice. Not so much about the sign-off header in particular (it’s not the only one).
Headers or trailers (?1) like “sign-off” (?2) is, in current
practice in projects like Git and Linux, effectively structured metadata
for the commit. These are all appended to the end of the commit message,
after the “free form” (unstructured) part of the body of the message.
These are token–value (or key–value) pairs typically delimited by a
colon and a space (:?).
Like I mentioned, “sign-off” is not the only trailer in current practice. See for example this commit, which has to do with “Dirty Cow”:
mm: remove gup_flags FOLL_WRITE games from __get_user_pages()
This is an ancient bug that was actually attempted to be fixed once
(badly) by me eleven years ago in commit 4ceb5db9757a ("Fix
get_user_pages() race for write access") but that was then undone due to
problems on s390 by commit f33ea7f404e5 ("fix get_user_pages bug").
In the meantime, the s390 situation has long been fixed, and we can now
fix it by checking the pte_dirty() bit properly (and do it better). The
s390 dirty bit was implemented in abf09bed3cce ("s390/mm: implement
software dirty bits") which made it into v3.9. Earlier kernels will
have to look at the page state itself.
Also, the VM has become more scalable, and what used a purely
theoretical race back then has become easier to trigger.
To fix it, we introduce a new internal FOLL_COW flag to mark the "yes,
we already did a COW" rather than play racy games with FOLL_WRITE that
is very fundamental, and then use the pte dirty flag to validate that
the FOLL_COW flag is still valid.
Reported-and-tested-by: Phil "not Paul" Oester <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Willy Tarreau <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>
In addition to the “sign-off” trailer in the above, there is:
Other projects, like for example Gerrit, have their own headers and associated meaning for them.
See: https://git.wiki.kernel.org/index.php/CommitMessageConventions
It is my impression that, although the initial motivation for this particular metadata was some legal issues (judging by the other answers), the practice of such metadata has progressed beyond just dealing with the case of forming a chain of authorship.
[?1]: man git-interpret-trailers
[?2]: These are also sometimes called “s-o-b” (initials), it seems.