LIGO (Laser Interferometer Gravitational-Wave Observatory) looks for ripples in the space-time continuum, gravitational waves, created by very violent events in the universe, such as black hole mergers. Albert Einstein predicted the existence of these gravitational waves in 1916. Technology has not advanced to a point where we can detect gravitational waves. LIGO has the possibility to test Einstein's general relativity theory.
Advanced LIGO is designed to improve the sesitivity of initial LIGO by a factor of 10. It is expected that Advanced LIGO sees gravitational wave sources possibly on a daily basis.