A lunar node is one of two calculated intersection points between the Moon’s orbital plane and the ecliptic plane. The ascending or north node is where the Moon crosses in one direction; the descending or south node is the opposite point.
Nodes are mathematical points, not planets or physical objects. Different calculation conventions can use a mean node or a true node, so a chart or ephemeris should state which one it provides.
Astrology gives the nodes symbolic meanings involving development, familiarity, or direction. Those meanings are reflective traditions, not scientifically demonstrated effects of the intersection points.
Sources and further reading
See the project methodology and pinned Astronomy Engine JavaScript reference for the project’s calculation boundary. Those sources do not assign the nodes’ astrological meanings.