Your reliability rating reflects how consistently you keep your assigned jobs on your schedule. Open the menu and it sits at the top next to your name, together with your other two ratings. This rating is mainly affected by cancelling your assigned jobs, and cancelling a job closer to the pickup time has a bigger impact. Your reliability rating is improved by winning and completing more jobs without cancelling them. See more about the reliability rating in Callout incidents.
A higher reliability rating helps you apply to more jobs and jobs that are further into the future. It does not affect which jobs you'll win after applying, only which jobs you can apply to.
If a shipper selects you as a preferred driver, you can apply to all of their jobs at any time regardless of this rating.