This type of insurance status is usually only available for people who are already in the public health insurance system in Germany (or anywhere in the EU). You can choose to go with the system even though you don't have a job, because you previously were covered by the system.
It is nearly always the best option to do this if you can. It usually arises when someone either moves back to Germany after some time abroad and is looking for a job, or if someone from an EU country moves to Germany and is freelancing or looking for a job.
If you're a freelancer from the EU click here.
If you're from the UK click here.