What does the ring signify to Cesario?

Malvolio has caught up with Viola (still disguised as Cesario). Malvolio gives Cesario the ring that Olivia has sent with him, rebuking him for having left it with Olivia. Viola realizes Olivia’s deception and plays along with it, pretending that she did indeed give the ring to Olivia. She tells Malvolio that Olivia took the ring and insists that Olivia must keep it.