All along, Melisandre has worn a choker with a giant red stone in the middle of it. The origins of the choker have never been fully revealed on the show, in the books whenever she’s performing her “magic” the stone in the middle of it glows (we’ve seen it glow on the show a few times, too, and it’s actually always glowing as we’ll later learn in the episode). Just what the choker is and how exactly it works is unclear, but we do know it’s able to grant her young age and beauty while she’s wearing it. Melisandre is incredibly vain and relied heavily on glamor to get whatever she wanted. The choker helps her do that because it often lowered the guard of men who she’d trying to influence or overpower (but it doesn’t always work — Jon rejects her advances).