Gravitational waves (GW) are vibrations of space-time propagating in the Universe and produced by extreme cataclysmic events. Passing GWs contract and expand space-time so that two points of space otherwise at rest have the distances between them shortened then increase.

This effect is very small and this perturbation is typically about a hundred million times smaller than the typical size of an atom. On Earth, we detect the relative displacement via gravitational-wave detectors; we use kilometer scale interferometers, which use a combination of mirrors and lasers to make precise distance measurements. There are a number of kilometer scale detectors in operation including LIGO-Handford and LIGO-Livingston, in the United States, Virgo in Italy, GEO in Germany, and Kagra in Japan.
Up until now, we have detected GWs from stellar compact events: the merger of compact objects in binary systems involving either two black holes (BBH), two neutron stars (BNS) or a mix of them (NSBH). With the gravitational signal, we have direct access to the distance (so the compatible host galaxy) of the event and the masses (indicator of the nature) of the two compact objects involved in the merger. BNS and NSBH mergers are the most promising sources of light counterparts as a fraction of the matter can be expelled and radiated through several mechanisms post-coalescence.
On September 14th, 2015, both Advanced LIGO detectors in the USA, H1 in Hanford, Washington and L1 in Livingston, Louisiana, made the first direct measurement of GWs. The event, GW150914, was determined to be the merger of two black holes, with masses of 36 M⊙ and 29 M⊙, into a black hole of approximately 62 M⊙. 3.0 solar masses of energy (5.4 × 1047 J) were radiated in GWs. The gravitational waves from this event, which occurred at a distance of 410 Mpc (1.3 ×109 light years), changed the separation between the test masses by 4×10−18 m, about one 200th of a proton radius. Since this first detection of GWs, there were 11 confirmed detections during first and second observation runs (2015-2017) and 56 candidates detected during the third observing run (2019-2020). The first results of the latest run have already shown the huge potential of gravitational waves to reveal new astrophysical events : the number of interesting candidates, including the two neutron star systems, potentially the first black-hole–neutron-star merger and asymmetric collisions.

Gravitational waves explained using stick figures
LIGO – What is a Gravitational Wave?
GW 150914: The first observation of a gravitational wave signal
The last results of the LIGO-Virgo publications
Warped Spacetime and Horizons of GW150914
Les ondes gravitationnelles ou les frémissements de l’espace-temps