It means GRADED by KWPN. Any breed can be KWPN graded. But it is generally recognised as perhaps the toughest breeding standard in the world, so if you've got a KWPN graded horse it should be a good one.
A warning! The true Dutch KWPNs with the huge trot movement can be VERY slow to grow up mentally and VERY difficult to manage until they are. I was warned and still bought one - still waiting for him to hack out alone after three years!