When two goods are complements, it means that they are often used together. For example, peanut butter and jelly are complementary goods because people usually eat them together. This is different from two goods that are substitutes, which means that people would use one instead of the other (such as coffee and tea).
There are a few things that can happen when two goods are complements. First, an increase in the price of one good can lead to an increase in the demand for the other good. This is because people will still want to purchase both items, but may be willing to switch to a cheaper alternative for one of the items if the price goes up too much. Additionally, an increase in the income of consumers can lead to an increase in demand for both complementary goods. This is because people will have more money to spend on all kinds of products, not just necessities. Finally, changes in fashion or trends can impact the demand for complementary goods. For example, if a new style of clothing becomes popular, people may start buying more clothes overall as well as accessories like jewelry and purses to go along with their new outfits.