Primaries are the flight feathers at the outer edge of the wing. You can think of them as being the feathers on the hand as opposed to the feathers on the arm, which are known as secondaries.

Most birds have 10 primaries but grebes, storks and flamingos have 11. The Passerines or perching birds basically have 10 primaries but the 10th, outermost, primary is reduced to a greater or lesser extent in some families and may have disappeared altogether in the case of the 9-primaried Oscines (Finches, Buntings, Tanagers and others).