This is not a CD. It is a piece of aluminium foil laminated with plastic.
Before the wine is poured, the foil is folded to fit into the neck of the bottle, like so.
This flexible disc helps to ensure that every drop of wine goes into the glass, and when you stop pouring there is none of that backflow that runs messily over the rim.
Clever ain't it? Of course, wine buffs would already know about this, but umami is very ignorant about wine, so this was an exciting discovery for her.
