minus over-correction vs plus under-correction

Hello everyone,

I am corresponding with an OD over email. He said:

"I would NOT like her to have -1.00 lenses with -1.50 astigmatism... I am OK with her having plus undercorrection to plano -1.50 in both eyes...
...
When the PO suggests minus lens over-correction or surgery... that is the time to make the trip to see me. "

Can you please explain

minus over-correction
plus under-correction

Thank you.

minus over-correction vs plus under-correction