ps-rollout is proprietary software developed by PlanetScale for use with the PlanetScaleDB Operator for Kubernetes.
ps-rollout is a tool that can be used to apply edits you have made to the YAML (or CRD) that controls the configuration of your PlanetScale cluster.
One update strategy is Immediate, which means that as soon as the Operator detects the CRD edits, it tries to apply them as quickly as possible. It assumes that your application can deal with those changes. This is often not the best approach to apply changes to your cluster, especially in a production or staging environment; and is not recommended by PlanetScale. It may be appropriate for a development or testing environment, however.
For the PlanetscaleDB Operator ps-rollout offers a more elegant solution. Rather than having the update strategy set to Immediate, the update strategy of External (the default) allows edits to the CRD to not be applied immediately. More information on how ps-rollout works is here.