What is the 'Rollup Batch - Hourly' Scheduled Job?

The 'Rollup Batch- Hourly' job checks whether any batch rollups need to be run (scheduled, manual run, asynchronous triggered), similarly to the old 'Rollup Batch Agent' job. The 'Rollup Batch - Hourly' job should always exist, otherwise your rollups will be impacted.

You can tell whether the job exists by either viewing the All Rollups tab and ensuring that there is text that says 'Running As...' near the top left of the interface or by checking the Scheduled Jobs list. If the job has been aborted, you can spawn a new job by clicking 'Run As...' from the All Rollups tab on the user you would like the rollups to run under.