A Nexus measures how much concentration an elf has; when the meter is full the cuff can be unlocked, though in Neverseen, Alden uses a small disc to push their meters to full, and Dex says that he was tempted to do that himself.
Biana Vacker is the youngest elf to get her nexus unlocked, younger than the previous elf, Fitz Vacker, her brother. Sophie used to wear two nexuses because Elwin was afraid of her fading away after when she almost faded one time.
Nexuses are described as a cuff with a crystal set in the center. Like many Elvin things, they are typically decorated and intricate. Nexuses usually contain jewels and crystals.
The force field a Nexus creates when an elf leaps can be tracked, which is why they dispensed of them in Neverseen.