This article contains spoilers. Read at your own risk!
It might spoil your gaming experience when first playing the related games.
The Quarantine Zone is a location within City 17 visited by Alyx Vance in Half-Life: Alyx. It is closed off from the rest of the city by a Combine Smart Barrier due to the severe Xen infestation inside. The area harbors The Vault which hovers right above it.
The buildings and open areas of the Quarantine Zone are covered by Xen fungus-like growth. Headcrabs, zombies, barnacles, antlions and Jeff are among the creatures that can be found there, including variations of these creatures never seen before.
The Combine appears to be trying to contain the Xen infestation in certain buildings (notably the Northern Star Hotel and the distillery) by covering the Xen flora with an orange substance, which seems to be extracted from Antlions. This work is performed by a specialized unit of Hazmat Workers with yellow uniforms.
The Quarantine Zone includes a subway station, a Vortigaunt lair, the Northern Star hotel, a distillery and a zoo, among other areas. The Zone is entirely abandoned due to its harsh conditions and dangerous wildlife. The only people found there, besides Combine troops, are the Quarantine Zone Vortigaunt and the scavenger Larry.
In Half-Life: Alyx, the train is stopped just outside the Zone due to the Combine shutting down the area, forcing Alyx Vance to make her way on foot and fight through the various Xen wildlife that has overtaken the buildings. Along the way she encounters a brain-damaged Vortigaunt, who cryptically warns her that her father is fated to die and that she will not be able to prevent it on her own. He also informs her that the Combine have many of his kin trapped and that she needs to rescue them and that she will need to follow “the Northern Star.”
Alyx continues through the Quarantine Zone, where she acquires a flashlight and fights off various headcrabs hiding in the dark tunnels. After fighting off some Combine soldiers she arrives at the junction where she can intercept her father's train, but discovers the controls are broken. She then uses a nearby switch to stop the train by leading it to a dead-end, derailing it. Inside she finds her father, who she nearly loses when he falls into a pit, only to be rescued by the Vortigaunt from earlier. He explains that he and Russell discovered images of a building containing what appears to be a Combine superweapon, contained in a massive ship called "The Vault" floating above the Quarantine Zone. He sends Alyx to disable the Vault while the Vortigaunt reminds her to follow the “Northern Star” and escorts Eli back to safety.