In exercise, we are encouraged to "strengthen our core" but what is our core?

I have a general idea, but I have never heard an actual description as to “what is our core.” I think I am in the generation just after this being a commonly used concept and I can find all sorts of information about exercises to strengthen my core, but I am just not sure what my core is. I am in PT now for my ankle (balance and strength are the focus) and the physical therapist keeps making reference to strengthening my core. I just shut up and do what she tells me, but I realized this morning that I really don’t know what she is telling me . . . .