Rahu and Ketu
Rahu and Ketu are the two points where the Moon's orbit crosses the Sun's path; Vedic astrology treats them as shadow planets.
What they are
The Moon's orbit is tilted against the Sun's apparent path. The two crossing points are the lunar nodes: Rahu, the north node, and Ketu, the south node. They always sit exactly opposite each other and move backwards through the zodiac, taking about 18 years for a full circle.
How the tradition reads them
Rahu is read as appetite, excess, speculation and the new; Ketu as detachment, loss and the old falling away. Both count as malefics. In mundane astrology Rahu is linked with bubbles and sudden hype, Ketu with sudden drops of interest.
In AstroFinex
The nodes are part of the engine's chart and dasha calculations, and eclipses are found where New and Full Moons fall near them.