Side meat is slabs of meat taken specifically from the sides of a pig, practically a staple in the deep south. May be smoked and cured, in which case it becomes known as bacon, or salted, in which case it is known as salt pork. Sometimes referred to as "side pork," as well, it is an indispensable ingredient for the southern dish of boiled greens.
Ma' Joad fried these up as simple dishes for her family. I would assume the fat and grease filled their stomachs.
Simple way to cook them up was just to throw them in a hot frying pan and let them fry to a crisp brown or however you'd like.