I am assuming you are referring to a fitness certification.
If you are referring to a fitness certification, then that would depend upon the type of fitness certification you possess.
If it is a group fitness certification, then you can teach group fitness.
If it is a personal trainer certification, then you can train people on a one to one basis.
Whatever certification you decide to get, though, you need to be sure that you visit the website of the organization who will certify you and learn more about the certification.
Hope this is of help to you.