How can one pay their debt if they are locked up?

The time in jail is a certain amount per day off the fine.
It is the ultimate end result, fines are to replace jail time.
If you don't pay them, or pay them off, you can also arrange to do community service.
If you fail to do any of these eventually you will end up in jail but most of the time when you get arrested the judge will allow you to do community service to stop clogging up the jails.