Your core muscles are the ones in the abdominal-lower back area of the body. Pretty much everything above the waist and below the pectoral muscles. Crunches are good for the abs, back raises (that device where you secure your legs and your torso is free to bend up and down at the waist) are good for the back, and you can turn sideways to work whatever the muscles to the sides of your abs are called. I call them the Abercrombie muscles. Anyway, a stronger core can help alleviate back pain, as will proper stretching