The phrase "to freeze the milk" means to put the milk in a very cold place, like a freezer, so that it becomes solid and doesn't spoil quickly. Full definition
Did you forget about a bottle of milk in the back of the fridge or a bag of frozen milk in the freezer? (pumpables.co)
Later, when I worked for a company that required a long commute, I got in the habit of freezing my milk in the company's large refrigerator / freezer combo. (natural-parenting-advice.com)
You start pumping to build up a stash of frozen milk for your return to work. (huffingtonpost.com)