What do forward and reverse primers do in PCR?

During the denaturation step in PCR, two complementary single DNA strands are released. The forward primer binds to the template DNA or the antisense strand of the DNA, while the reverse primer binds to the non-template DNA strand or the sense strand of DNA. 

Using both, forward and reverse primers is important for PCR to be successful. Exponential amplification cannot occur if only one primer is used because in this case, only one strand of dsDNA would get amplified, resulting in only one copy being produced per cycle.