Copyright is a series of rights an author has over their journal articles, which may be given away in writing, in whole or in part, to reproduce, to distribute, to make derivatives, to publicly display, or to deposit in any personal, institutional, or open access, digital repository or archive. When you decide to publish in a journal, typically an author signs a publishing agreement that hands over to the publisher all of these rights. You should read the publication agreement carefully or check the publisher's website advice page for authors to check for wording that permits you to archive your work and/or retain rights in order to meet funder mandates as with the NIH Public Access Policy.
The NIH suggests that authors include wording in the Submitted Manuscript, and if the article is accepted, in the Author Accepted Manuscript as well:
This manuscript is the result of funding in whole or in part by the National Institutes of Health (NIH). It is subject to the NIH Public Access Policy. Through acceptance of this federal funding, NIH has been given a right to make this manuscript publicly available in PubMed Central upon the Official Date of Publication, as defined by NIH.
GW authors can retain their copyright permissions using either the GW Addendum to Publication Agreement or the SPARC addendum in order to comply with either funder mandates, such as the NIH Public Access Policy or the institutional policies, such as the GW Open Access Resolution. In general, the University recommends that faculty authors not assign exclusive copyright to publishers.