The sign-off is a simple line at the end of the explanation for the patch, which certifies that you wrote it or otherwise have the right to pass it on as an open-source patch. The rules are pretty simple: if you can certify the below (from developercertificate.org):
Developer's Certificate of Origin 1.1
By making a contribution to this project, I certify that:
(a) The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or
(b) The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the same open source license (unless I am permitted to submit under a different license), as indicated in the file; or
(c) The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it.
(d) I understand and agree that this project and the contribution are public and that a record of the contribution (including all personal information I submit with it, including my sign-off) is maintained indefinitely and may be redistributed consistent with this project or the open source license(s) involved.
The DCO requires a sign-off message in the following format appear on each commit in the pull request:
Signed-off-by: Random J Developer random@developer.example.org
using your real name (sorry, no pseudonyms or anonymous contributions.)
The DCO text can either be manually added to your commit body, or you can add either -s or --signoff to your usual git commit commands. If you forget to add the sign-off you can also amend a previous commit with the sign-off by running git commit --amend -s. If you've pushed your changes to Github already you'll need to force push your branch after this with git push -f.