A Certification is a course - or collection of courses that are repeating ie. expire at a pre-set time and need to be repeated after the certification is due.
An example of a certification could be level 2 CPR revalidation
To get the initial CPR endorsement you would need to complete the following courses
CPR theory - Course 1
CPR practical session (face 2 face session) - Course 2
Then every 12 months - to keep your CPR endorsement valid, you will need to repeat course 2 - the CPR practical session. If the session is not completed in time - then the CPR endorsement expires. Reminders are sent out automatically to prompt people when it is now time to complete the required course again.
Please note: You cannot complete a Certification early, as this is counted as revision and not a new attempt.