It is a great circle on the celestial sphere representing the Sun's apparent path during the year, so called because lunar and solar eclipses can only occur when the Moon crosses it.
In plain language
The night sky is studded with fixed stars. To an observer on the Earth, the Sun (yellow ball) travels among these fixed stars, one full circle every year. The Sun repeats the travels among the same stars year in year out. Color the path with a yellow line, and the yellow line is the Ecliptic. The Chinese name it Yellow Path (黃道).
The lunar nodes are the orbital nodes of the Moon – the points where the orbit of the Moon crosses the Ecliptic. The ascending node is where the moon crosses from south of the ecliptic to north of the ecliptic. The descending node is where it crosses from north of the ecliptic to south of the ecliptic.
In plain language
The Moon (white ball) revolves around the Earth, completing one revolution in 29.1/2 days. The Moon’s orbit (around the Earth) is inclined 5 degrees 8 minutes to the Ecliptic. Thus in every Moon month of 29.1/2 days, the Moon crosses the Ecliptic on 2 occasions – Once from south to north (the point of intersection is called ascending node), and once from north to south (the point of intersection is called descending node). The ascending node and the descending node are separated by 1800. Color white the points of intersection of the ascending node, and it is the (invisible) North Node in the sky. The Chinese name it the White Path (白道).
Corresponding names of the lunar nodes in 3 languages
North Node (ascending node) = Hindu Rahu = Chinese 曪睺
South Node (descending node) = Hindu Ketu = Chinese 計都
The positions of the North Node are always tabulated on the Ephemeris, but not the South Node (which is opposite to the North Node). The North Node moves in clockwise (i.e. backward) motion, completing one celestial cycle of 3600 every 18.60 years.
The North Node is marked on our Color Ephemeris with thick broken green line. Heavily stressed North Node is associated with tumultuous financial markets, e.g. in January 2015 (Devaluation of Russian rubles, soaring US dollars, decoupling of Euro-Swissy, Greek bankruptcy …)