The Ringer - S1-E1
Continuity mistake: As Link is walking towards his window after getting out of bed, there's a shot showing two of the windows in his room. There are block-like extensions under the windows - a ledge with rings on them. In this shot, they are not the same. The one on the right is significantly bigger and taller than the one on the left. In the rest of the episode they are pretty much uniform in shape and size for all windows in that room. (00:01:35)
Answer: It's the key to her apartment. She wears it around her neck so she doesn't lose it.