A bit of history: Moon's predecessor Park Geun-hye was impeached and removed from office for corruption. As the Post notes:
Moon’s victory will bring an end to almost a decade of conservative rule in South Korea and the hard-line approach toward North Korea that had Seoul walking in lockstep with Washington.While Trump has indicated we are out of patience with North Korea, Moon is known to prefer dialog with the leaders of the North. Whether conflict between the U.S. and South Korea will ensue is unclear; Murphy's Law suggests it's inevitable.