The sagittal abdominal diameter is basically a way to measure how big your belly is from front to back.
Think of your belly like a balloon. If you blow up the balloon too much, it would become bigger not only from side to side, but also from front to back. The sagittal abdominal diameter is like measuring the size of the balloon from the front to the back.
Doctors use this measurement to figure out if someone has too much belly fat. Having too much belly fat can be bad for your health, and can increase your chances of getting things like diabetes or heart disease.
So, when your doctor measures your sagittal abdominal diameter, they are really just trying to figure out if your belly has too much fat or not.