Most criminal defendants are legally indigent and can’t afford to pay for an attorney. On the other hand, the state can’t legally prosecute indigents unless it provides them with an attorney. To satisfy this requirement, many states have set up public defender offices.
A "panel attorney" is a lawyer who signs-up with the court to handle indigent cases when the public defender's office cannot represent the client. In some counties, where there is no public defender office, all indigent defense work is handled by panel attorneys. Being a panel attorney is a good way to have a steady stream of work, and payment (from the courts) is reliable.