DNA polymerase is an enzyme that facilitates DNA replication. After the DNA double helix has been "unzipped" to initiate replication, polymerase traverses each strand in the 3' to 5' direction and adds complementary bases at each position, thus creating a new strand (in the 5' to 3' direction) and doubling the total amount of DNA.
Some polymerase molecules even have the ability to "proofread" themselves, meaning that the polymerase can detect if a non-complementary base is added to the template strand of DNA and will replace it with the correct base; see the figure below for an illustration.
