A viscous, milk-colored, alcoholic beverage made from the agave americana plant.
It became a meme based on a commercial aired during the broadcast of the final episodes of Mahou Shoujo Madoka Magica on April 22, 2011. The commercial can be seen here. The narrator asks "謎の白い液体の正体とは？" (What is the identity of this mysterious white liquid?). And, of course, only fanartists dare to show us the answer.
Meme-related Pixiv tag: 謎の白い液体 (nazo no shiroi ekitai)