The series is named after a real challenge in celestial mechanics:

The books are known for exploring complex concepts like nanotechnology, cryonics, higher-dimensional physics (Sophons), and the "Dark Forest" theory of cosmic sociology. Scientific Basis

An alien race living on a planet with three suns. Because the gravitational interaction of three suns is mathematically unpredictable (the "three-body problem"), their world suffers from extreme "Chaotic Eras" that periodically destroy their civilization.