The core is made up of several muscles that help stabilize the core and perform trunk actions. The external obliques pair with the internal obliques and the transverse abdominus to make up the lateral abdominal muscles.  The external obliques perform several actions. When contracting on the same side they perform lateral flexion of the trunk, or side bending. When they contract both sides together they flex the trunk forward,  and it also helps with trunk rotation when one side contracts.  The external obliques help with many daily functions including breathing, singing, and defecating. When contracted on both sides and in combination with other abdominal muscles increase intra-abdominal pressure.   A lot of times core programs focus on what we call the ‘six pack’, and while it is good to strengthen all muscles of the core, the rectus abdominis only performs flexion. The inner core muscles are much more important for stabilization, breathing, and daily activities. Unfortunately, they are often forgotten in exercise programs.  How to strengthen this muscle. A technique called ‘diaphragmatic breathing’ helps to create more efficient breathing patterns. Lateral flexion with weight on the same side is a great way to strengthen the external obliques. Another great way is to start on your back, reach towards your opposite hip and lift your shoulder blade off the floor. Make sure to breathe through these exercises. 
How to stretch this muscle. In a seated position, reach your arms above your head and clasp your fingers. Inhale and reach to one side pressing your fingers up and to the side. Do not go to the point of pain or instability. Hold for about 30s if you can and repeat on both sides.