Goods are complementary when their consumption is tied to each other. For instance, automobiles and tires: tires are sold because automobiles are sold and vice versa. The increase of the price of automobiles will cause fewer automobiles to be purchased, and thus, fewer tires as well. The relationship between the price of automobiles and the quantity of tires is inverse.
Tires and cars, bullets and guns, lamps and lamp shades, cream and coffee, nails and hammers, nuts and bolts, are all items that go together. They are complementary goods.