Okazaki fragments are short segments of DNA that are synthesized on the lagging strand during DNA replication. They are formed because DNA polymerase can only add nucleotides in a 5' to 3' direction, leading to the creation of these fragments as the replication fork opens up. These fragments are later joined together by the enzyme DNA ligase to create a continuous strand of DNA. This process is essential for ensuring that the entire DNA molecule is accurately replicated, allowing for proper cell division and function.