The purpose of the side stretch is to target the muscles between the ribs and the hip, which can get tensed after having spent a lot of time sitting down, or slept on a too soft madras. Another reason can be that one of your legs are slightly shorter than the other. This is an easy stretch exercise you can do both standing up and sitting down.
Sit straight and tense your abs slightly. Raise your right arm above your head and slowly lean to the left until you feel tension on your side. It’s ok to bend your arm to get deeper into the stretch. Don’t lift your butt from the chair or lean forward or backward. Hold the position for the desired amount of time and then switch to the other side.
You can also do this the side stretch while standing up. Stand with your feet shoulder width apart and raise your right arm above your left and slowly lean into the stretch. Hold the position for the desired amount of time and then switch to the other side.