Getting a health certificate means arranging for you to go to the restaurant for trial operation, which means you can go to the restaurant for two days and experience all the workstations. Let you know something about KFC's work and let KFC know something about your learning ability. There is also to let you feel the intensity of work here and see if you can eat.
The next interview is basically to let you talk about your feelings. Let the leaders in charge know something about you and finally decide whether to stay with you.
If you still don't understand, ask again.