What is the difference between a weak spot in the abdomen wall and a hernia?

Spectrum of disease. A hernia is a hole in the abdominal wall through which intra abdominal contents can protrude, and appear as a lump under the skin. These holes sometimes start out with a thinning of the muscles, which is more typical for inguinal (groin) hernia. The weakness does not allow passage of abdominal contents, thus is not dangerous. It may however be a precursor for a hernia.
Matter of degree. A weak spot in the abdominal wall is on its way to becoming a hernia. Sometimes this happens suddenly and sometimes gradually.