During pregnancy, the baby expands the abdominal muscles as it grows and develops. This stretches the two main muscles of the abdomen, called the rectus muscles, and pushes them apart. This can also be caused by excessive weight gain. As the muscle is pushed and stretched apart, a thin layer of tissue forms between the two muscles. When the muscles are stretched apart in this way, it is called a "diastasis" of the muscles.
After pregnancy, the muscles remain stretched and are no longer attached in the middle. All that remains between the muscles is a thin layer of scar tissue. This tissue is weak and allows bulging out of the abdomen. This happens after excess weight gain also. Unfortunately, no amount of exercise is able to bring these two muscles together again. Thus, the lower abdomen will continue to bulge unless the muscles are repaired.
During an abdominoplasty, these two muscles are sutured together, restoring them to their original positions. This techinque is called a "diastasis repair." When it is done, it brings the two edges of the muscles together, as they were before pregnancy or excess weight gain. This creates a flatter appearance of the muscle and gives the abdomen a more toned look.