According to Scitable, RNA primase is an enzyme involved in the replication of DNA strands during cell division. It goes over a single DNA strand and creates RNA sequences called primers, which transcribe DNA into RNA. These short sequences of RNA are complementary to the DNA strand they were hovering over, and serve as a template for a new strand of DNA.
DNA resembles a twisted ladder, with each rung made up of two complementary molecules which bond together yet remain distinct. Another enzyme, called DNA polymerase, goes over the RNA primers and adds the molecule required to complete each rung. A different kind of DNA polymerase proofreads the strand as it goes along to ensure that there are no errors. In this way, DNA is copied and new DNA chains are created which resemble the parent chains.
RNA primase acts in a discontinuous manner, never transcribing more than a few chunks of a long DNA strand. This creates "nicks" in the sides of the DNA ladder created by the primers, which are filled in by another molecule called "ligase."
Just as DNA polymerase creates new DNA strands, RNA primase is considered a kind of RNA polymerase because it creates new strands of RNA.