A very good video about explanation of gravitational waves
What are gravitational waves?
Colossal cosmic collisions and stellar explosions can rattle spacetime itself. Relativity rule predicts that ripples in the fabric of space-time radiate energy . After some time, they reach Earth, compress spacetime by as little as one ten-thousandth, the width of a proton.
How are they detected?
To spot a signal, LIGO uses a special mirror to split a beam of laser light and sends it down two 4-kilometer-long arms, at a 90 degree angle to each other. After ricocheting back and forth 400 times, turning each beam’s journey into a 1,600 kilometer round-trip, the light recombines near its source.