What are they?
In astronomy, Rahu and Ketu are not physical planets. They are mathematical intersection points known as the Lunar Nodes.
The Earth orbits the Sun on a flat plane called the Ecliptic. However, the Moon's orbit around the Earth is tilted by about 5 degrees relative to this plane.
Because of this tilt, the Moon's orbit crosses the Earth's Ecliptic plane at exactly two points.
The Ascending Node. This is the exact point where the Moon crosses the ecliptic going from South to North.
The Descending Node. This is the exact point where the Moon crosses the ecliptic going from North to South.
The nodes are not stationary! The Moon's orbital plane slowly wobbles, causing the nodes to drift backwards (retrograde). It takes exactly 18.6 years for Rahu and Ketu to complete one full cycle around the Zodiac. This is called Nodal Precession.
Eclipses: A Solar or Lunar eclipse can only happen when the New Moon or Full Moon aligns closely with either Rahu or Ketu. In ancient mythology, these nodes were personified as shadow demons that "swallowed" the Sun and Moon.