Perfectly competitive markets are those market's characterised
by large number of buyers and sellers selling homogeneous
products.
In long run these firms earn zero economic profits as they set
the prices of their goods and services equal to the marginal cost
of production.
This is because in short run, firms earn enough profits, which
attracts new firms into the market. Due to increased competition
the existing firms are forced to set P=MC in long run, thus earning
zero ecomomic profits.