Answer:
A "node" is a point where the Moon passes through the ecliptic plane (Earth's orbit around the sun). Because the moon's orbital plane around the Earth is inclined 5 degrees with respect to Earth's orbit around the sun, the Moon's orbit only intersects the Earth's solar orbit every 6 months (approximately).
Solar eclipses occur when the Moon touches a node on the line of nodes between the Earth and the Sun. A lunar eclipse occurs when the opposite happens - the moon touches the ecliptic plane when the Earth is between it and the sun.