“Where is Nagini?” said the cold voice.
“I I don’t know, my Lord,” said the first voice nervously. “She set out to explore the house, I think. . . .”
“You will milk her before we retire, Wormtail,” said the second voice. “I will need feeding in the night. The journey has tired me greatly.”
How would one milk a giant magical snake? Would milk actually be produced, or is milk just a verb that would actually produce another magical substance?
Snakes are "milked" by expressing the teeth against a rubberised (or flat) surface and then massaging the venom glands in order to collect the snake's venom. In the real world, this would then be used to produce anti-venom. In the wizarding world, Voldemort uses Nagini's venom to create his rudimentary body potion
Later we learn the use to which the venom was put;
Wormtail was able to follow the instructions I gave him, which would return me to a rudimentary, weak body of my own, a body I would be able to inhabit while awaiting the essential ingredients for true rebirth … a spell or two of my own invention … a little help from my dear Nagini’ – Voldemort’s red eyes fell upon the continually circling snake – ‘a potion concocted from unicorn blood, and the snake venom Nagini provided … I was soon returned to an almost human form, and strong enough to travel. Harry Potter and the Goblet of Fire
Milking a venomous snake means to remove venom from its fangs. As discussed in this Wired article.
