Acini di pepe is a small, round pasta shape from Italy, about 1-2 millimeters across, whose name means "grains of pepper" for its tiny, bead-like appearance.
Made from semolina wheat flour and water and extruded into its round pellet shape, acini di pepe is small enough to disappear into broth-based dishes like minestrone and pasta e fagioli without overwhelming the other ingredients. It also holds up well in cold pasta salads, since its compact shape resists going waterlogged when dressed.
The pasta appears beyond Italy as well: it's used in Greek avgolemono soup, simmered in a lemon-egg broth. It has a neutral flavor that takes on whatever it's cooked with, and it cooks quickly, in about 5-7 minutes.
Whole wheat and flavored versions (spinach, tomato) exist, but the small, round semolina shape remains the standard form.