U.S. Copyright law gives the author of an original work, such as a scholarly article, the exclusive rights to reproduce, distribute, adapt, publicly perform, and publicly display the work.....until you sign a written agreement to transfer some or all of these rights.
This brochure from SPARC gives a concise explanation of:
Attach an author addendum to your publishing agreement in order to specify which rights you want to retain as the author.
Here are some templates and online tools to help you generate a personalized addendum.