That exercise is meant to strengthen mostly our abdominal muscles, located in the abdomen (muscles in front of our stomach).
A easier way to identify the abdominal muscles would be by saying that those are the muscles where the famous six-packs form. Six-packs are an accentuation on the abdominal muscles, that make poeple look like they're wearing six packs under their skin.
The muscles in read on the picture down below are the abdomminal muscles. The second picture is just so you can see how the acentuation looks from outside (six packs).
Hope it helped,
Happy homework/ study/ exam!
