DNA replication occurs in a pre-determined direction which occurs in the reverse direction in the two strands. In one of the new strands, the replication fork therefore advances in the opposite direction to replication. As a result, replication must be carried out in several stages, each leading to a small fragment of DNA produced: the Okazaki fragment.
Diagram of a replication fork. The new strands are shown in dark blue. The primers are shown in pink and the arrows on the lower strand show the Okazaki fragments. © Gluon, Wikimedia CC by-nc-sa 3.0