The structure indicated is the quadratus lumborum muscle.
The quadratus lumborum muscle is one of the muscles which makes up the posterior abdominal wall.
There are four muscles which contribute to the posterior abdominal wall:
Origin: Transverse process of L5, iliolumbar ligament, iliac crest.
Insertion: Transverse processes of L1-L4, inferior border of 12th rib
Action: Depresses and stabilises 12th rib. Weak lateral flexion of trunk
Innervation: Anterior rami of T12-L4
Learn more about the muscles of the posterior abdominal wall in this tutorial