Leeks are a type of vegetable from the same family of produce as onions. They have long, stem-like bundles of leaves that are edible.
The leek is an item that has become commonly associated with the virtual idol Hatsune Miku by fans. As such, many songs featuring her as the singer reference leeks in some manner. Leeks are also commonly included in Miku merchandise, as well as in video games that feature her.