Wearable Lanyard
You’d think that since people are around their phones so often, they’d have all the knowledge regarding every kind of smartphone accessory there is in existence. When the OP purchased a smartphone case, they also received a strange round object with a long rope on one end. Is this some kind of advanced tracking device or something?
Nope, it’s nothing but a simple lanyard that you can wear around your neck. You simply run the tinier rope through a small loop on the corner of your phone, then run the entire lanyard and phone through the smaller rope. Phone lanyards were more popular before smartphones when Nokia and Sony Ericsson were still around.