Subjugates, also known by the archaic term darklings, are humans who have consumed enough vampire blood to Turn and are considered half-turned. They are typically mundanes kept around by vampires to feed on when they're short on blood. These subjugates are loyal to the vampire they serve, drawn to them and hoping to become vampires themselves once they die.
When a vampire decides it prefers a ready supply over an occasional snack, vampires will create a vampire subjugate—like "walking snack machines", as Isabelle Lightwood put it. They will start feeding its bitten human small amounts of vampire blood to keep it docile and connected to its master. Subjugates worship their masters, and devote their lives to serving them, wanting to be near them at all times. They follow their every command and finds offense when others speak badly of their masters.
This vampire blood in them makes them irresistibly drawn to vampires, particularly the vampire whose blood they have in their system, some begging to be Turned. Anxiety in the sun and when seeing vampires, even in movies, are some signs. Although the effects are supposed to fade or wear off over time, most are drawn to the place where they received the blood, or the vampire from whom they ingested the blood. They begin to feel as if they are turning into vampires themselves, mainly because this is how vampires pass their powers to others—through blood exchange. The turning process will then only continue if the darkling is killed, or bitten by a vampire and drained of blood ("blood death"). Because of the healing and transformative properties of vampire saliva, most darklings survive this and become fledglings. When turned, subjugates lose their obedient and worshipful nature when they are turned into vampires themselves.
Instead of becoming weak from blood loss all the time, the healing properties of vampire saliva helps them live longer—increasing their red blood cell count, making them stronger and healthier, albeit still making them seem skinnier and paler.
Human subjugates do not eat food. They live on a mixture of vampire and animal blood, keeping them in a state of suspended animation. They are not immortal, but, due to the vampire blood and saliva in their system, they age very slowly.
Holy water is also known to counteract the effects of the consumed vampire blood. It must be taken in great amounts, inducing the vomiting and coughing up of the vampire blood out of their system.
It was once not against the Covenant Law for a vampire to feed on a human, as it doesn't really hurt them, but eventually, it became illegal. The creation of new subjugates was made illegal in 1962, at the Seventh Accords. Vampires who had subjugates created prior to that were allowed to keep them—such as Camille Belcourt's subjugates—as well as to transfer existing subjugates to other vampires.
Will Herondale almost becomes a darkling after he bites and swallows some blood from De Quincey in Clockwork Angel. He then drinks buckets of holy water, making him sick enough to cough up de Quincey's blood.