Each product that exists can be classified into a market in which it competes with other products for the attention of consumers. For instance, apples might be considered a snack-fruit and compete with items including pears and bananas. In other words, if the price of apples increased then demand would substitute towards one of these other products. The market for apples, would thus include these other fruits which substitution can easily happen.
Such substitution means that apple producers are restricted in their ability to increase prices. Increasing prices too much would result in this demand substitution towards other products.
On the other hand, other products might have a more restricted market. [...]