I don't know. Whatever pasta's made of. Probably semolina. There are several different recipes for it, just like with any pasta.
The OED says this: "The grain of the African Millet, Holcus spicatus Linn., Penicillaria spicata Willd., a cereal indigenous to Africa, where it has constituted from the earliest times an important article of food." first recorded use around 1600. Entry for "millet" as follows: "A graminaceous plant, Panicum miliaceum, native of India but extensively cultivated as a cereal in the warmer parts of Europe, growing three or four feet high, and bearing on a terminal spike or panicle a large crop of minute nutritious seeds. a. The grain." That seems different than semolina (my speculation above), which is made from wheat. ... Maybe that's just another way to make it? I'm seeing a lot of websites showing it made from semolina too.