A back promotion is a process that allows you to promote user stories from higher environments to lower environments.
With a back promotion you can minimize differences among orgs from a metadata perspective. Once the approved user stories are successfully promoted to a higher environment, you can back promote the changes to other lower environments to keep them in sync with upstream environments.
It is recommended to back promote changes to reduce merge conflicts. When user stories have passed all the quality gates and their changes are being promoted from a lower environment into a higher environment, it is a good practice to merge the same set of changes back to other lower environments in order to minimize the differences and reduce potential conflicts.