Primers are small DNA sequences that are designed to start DNA replication in a laboratory technique called polymerase chain reaction, or PCR, to amplify certain segments of DNA. Forward and reverse primers differ in the direction in which they initiate the replication.
DNA strands are complementary to each other; while replicating DNA, these strands are separated. Forward primers are usually attached to one of the strands to allow DNA synthesis towards the reverse primer. The reverse primer is designed to attach to the complementary strand to synthesize DNA in the reverse direction — towards the forward primer. The primers are added to PCR experiments to initiate the process of replication by providing the initial nucleotides to the new strand.