It's stitching done at the ends of the bones of corsets. It's both decorative and functional. It really is pretty and you can see some really gorgeous examples from the 19th century. But it also prevents the ends of the bones from rubbing through your corset, keeps them at the right length within your corset, and prevents them from twisting in their channels.
This corset has flossing that looks like feathers: https://www.pinterest.com/pin/273945589806770894/
This corset has flossing in Xs with an extra little floral spray: https://www.pinterest.com/pin/554787247823038746/