The author is the copyright holder.
As the author of a work you are the copyright holder, unless and until you transfer the copyright to someone else in a signed agreement.
Assigning your rights matters.
As author of a work, you possess the exclusive rights listed above unless and until you transfer them. If you transfer copyright ownership without retaining these rights, you must ask permission of the new copyright owner to use your work.
The copyright holder controls the work.
Decisions concerning use of the work, such as distribution, access, pricing, updates, and any use restrictions belong to the copyright holder. Authors who have transferred their copyright without retaining any rights may not be able to place the work on course web sites, copy it for students or colleagues, deposit the work in a public online archive, or reuse portions in a subsequent work. That’s why it is important to retain the rights you need.
Transferring copyright doesn't have to be all or nothing.
The law allows you to transfer copyright while still keeping rights for yourself and others. The terms of your publication agreement determine what rights you give to your publisher and what rights you retain.