It's a terrible system that only Southworst uses, and SWAPA refuses to get rid of. It reduces real captain upgrade slots and allows the company to cover captain trips at normal pay that they otherwise would have to pay premium pay to a real captain to fly.
Basically, the top of the FO list can bid to get their 4 stripes, but then they only fly as captain when there are trips for them to pick up. The rest of the time they're flying their normal FO trips until they can hold a real captain bid later.