This is a good stretch for your chest and lateral core muscles. Having a strap or something to hold can help you get deeper into the stretch, but it’s not necessary!
Hold a strap (or perhaps a rolled up long towel, etc) in your hands and roll your shoulders back to open up your chest. Press back against the strap while keeping your arms straight to get a good stretch across the chest. If you want a slight abdominal stretch, you can arch your back and lean backwards.
Then, slowly lean your torso to one side while keeping the arms straight to stretch your side. Repeat on the other side!