As the author of the thesis, you are the copyright owner. Although not required by U.S. Copyright law, it is good practice to include a proper copyright notice on the thesis, as in the DI template.
You may choose to register your copyright claim with the U.S. Copyright Office. Circular 1 from the Copyright Office explains the benefits of registering your claim.
Licenses are permission the creator gives to others to use their work in ways that would otherwise not be permitted by copyright law. Licenses are permission; they are not a transfer of copyright. DI requires that you give a nonexclusive fully paid license to the college to put your thesis in the DI digital repository, on the DI website and other appropriate locations, and to make them available to DI students, faculty and staff and to others upon request. This means that DI can distribute master’s theses in a way that allows other scholars to read it and use it for noncommercial purposes, as long as they do not make changes to the work and always give credit to the author. This license is designed to enable scholarship and to protect you from plagiarism.
Since this permission granted to DI does not transfer copyright ownership, you continue to hold all of the rights in the copyright 'bundle' (the exclusive rights to reproduce, distribute, display and perform your work, as well as the right to authorize derivative works). These rights will now be subject to the license given to DI, but nothing in the license will prevent you from transferring your copyright to some other party at a later date if you wish to do so.
You may also choose to use a Creative Commons license to share your work with others. For uses beyond the established license terms or those uses permitted under law (such as fair use), researchers may contact you directly for permission.