According from Einsteins theory General theory of relativity space and time are combined into space time. It consists of three spatial dimensions and time. One way to imagine this to visualize a fabric stretched out tight. When something massive is place in this fabric it creates a well or depression in the fabric. The more massive the object in this fabric the deeper and steeper the well will be. if an object is orbiting another object the system will send out waves in this fabric. The more massive these two objects are the larger the waves will be. Such as two black holes colliding, in this case each about 30 solar masses, with diameters with a diameter about 150 km. this collision took place about 1.3 billion years ago.
This collision was detected by two observatories one in Louisiana, and Washington state. In these devices, light is split into two beams that travel at 90 degrees to each other and then the light is reflected back to a detector. If there is a difference in how much time the light, it means the distance the light traveled is different so something stretched the tube the light is in. The difference in length was 1/1000 the diameter of an atom over 4 km so the signal is very weak, and there are many ways of trying to get rid of envromental noise.