One license option that is available to you when you contribute a resource is All Rights Reserved. This means that as the creator of the materials you are submitting, you choose to reserve all rights related to the reuse of the materials included in your resource. It is assumed, though, that by submitting your materials to the Sandbox you are inviting others to access, read, and be inspired by the materials you've shared.
All Rights Reserved is available to contributors to the Sandbox both to preserve the autonomy of those creating and sharing their resources through this platform, and to offer a license option that can be applied to resources created collaboratively by consortia and other large bodies--cases in which the contributor is not the sole creator of the materials, and thus does not have the right to assign to them a more open license.
All Rights Reserved is the default license on the Contribute a Resource form; however, you may also opt to assign to your resource a Creative Commons license, allowing colleagues in the profession more leeway in the ways they can reuse, adapt, and build upon your materials in their own work.
See also the ACRL Framework Sandbox Community Guidelines.