A Registration Certificate is a confirmation of your right to reside in the UK as an EEA national.
As an EEA national you have an automatic right to reside for 3 months in the UK, however after 3 months in order to continue to have a right to reside you need to be a qualified person.
You are a qualified person if you are a worker, job seeker, self employed, student or self sufficient person.
You can apply for a Registration Certificate if you are an EEA national and can prove that you are a qualified worker.
The Application is made to the Home Office and it is usually decided within a 6 month period.