The line "Who put butter in the White Watch?" is uttered by the Doctor in a playful and sarcastic way. It's a joke about a group of people who are being hunted by the Weeping Angels. The "White Watch" is a group of soldiers who are being controlled by the Angels, and the Doctor's comment is a humorous reference to the absurdity of their situation.
There's no actual butter involved, and the line is a metaphor for the way the Angels are manipulating their victims, turning them into "butter" for their own purposes.
The line has become a popular meme and a reminder of the Doctor's quick wit and ability to find humor even in the most dangerous situations.