The cows are a (bad) metaphor for the means of production, obviously. The metaphor sneakily puts you in the shoes of a capitalist and, whoa, it turns out capitalism is good when you are one of the few it benefits.
I can't believe I was once taught this bullshit in school.
You milk both them, your boss pays you as little wages as they possibly can and then you can pay the boss to buy milk.
That's just removing abstraction from an otherwise good explanation