The FSF (Free Software Foundation) is the copyright holder for GNU Emacs. The FSF is a nonprofit with a worldwide mission to promote computer user freedom and to defend the rights of all free software users. For general information, see the website https://www.fsf.org/.
Generally speaking, for non-trivial contributions to GNU Emacs and packages stored in GNU ELPA, we require that the copyright be assigned to the FSF. For the reasons behind this, see https://www.gnu.org/licenses/why-assign.html.
Copyright assignment is a simple process. Residents of some countries can do it entirely electronically. We can help you get started, including sending you the forms you should fill, and answer any questions you may have (or point you to the people with the answers), at the email@example.com mailing list.
(Please note: general discussion about why some GNU projects ask for a copyright assignment is off-topic for emacs-devel. See gnu-misc-discuss instead.)
A copyright disclaimer is also a possibility, but we prefer an assignment. Note that the disclaimer, like an assignment, involves you sending signed paperwork to the FSF (simply saying “this is in the public domain” is not enough). Also, a disclaimer cannot be applied to future work, it has to be repeated each time you want to send something new.
We can accept small changes (roughly, fewer than 15 lines) without an assignment. This is a cumulative limit (e.g., three separate 5 line patches) over all your contributions.