This picture is of the swimming pool that is located on the west side of the Stabian Baths. It measures 31 x 25 feet and is 4 feet deep. A small portion of the palestra is visible on the right side of the image.
The Stabian Baths were the oldest and largest baths in Pompeii and were constructed in the second century B.C. They included a (un)dressing room (apodyterium), a medium temperature room (tepidarium), a hot room (caldarium), and a cold room (frigidarium). One usually proceeded through the bath in that order. Besides these usual rooms, they included a men's and a women's sections (thus baths), a large open exercise area (palestra), and a swimming pool.
The earthquake of A.D. 62 severely damaged these baths and some areas were not in use at the time of the eruption of Vesuvius.