A Planetary Alignment (or Syzygy) is a straight line configuration of several planets. In the first Tomb Raider Movie, Lara Croft: Tomb Raider, an alignment of all nine planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) is imminent. Such an event is said to occur every 5,000 years.
Full Planetary Alignment
On May 15h 2001, the first stage of the planetary alignment occurs, in which Pluto and Neptune align with Uranus.
During this first stage, an ancient artefact, the All-Seeing Eye clock is activated and starts counting down to the full planetary alignment.
In his letter to Lara, Lord Richard Croft tells her that she has to be at the hiding place of the first part of the Triangle of Light, in the Tomb of the Dancing Light, in Cambodia, at the exact moment of the second stage of the alignment.
Only during this second stage can the first part of the Triangle be found, using the All-Seeing Eye as a key.
Real Life Facts
Such an event - where all nine planets (please note that nowadays we only have eight planets in our solar system, as Pluto is no longer a planet by definition) align - is highly unlikely and will not occur every 5,000 years.