Are you referring to exercise? Sometimes we say we are 'working our core' when we are doing exercises which work on strengthening the body (minus the arms and legs). If that is what you are referring to, "What is my core doing?" probably means to ask yourself what your body is doing when you perform a certain type of exercise. I hope this is helpful, Becky

By any chance, do you mean the word "corps?" The word "corps" in English is pronounced EXACTLY the same as "core" and means a large army unit with tens of thousands of soldiers in it.

Can you give more context to help us work out what this means? Is it to do with taking exercise?