consider some highly energetic process,massive accelerating bodies such as 2 neutron star revolving or merging of 2 massive black holes. These events cause disturbance that propogate much like waves on water that result from  throwing a stone. This disturbance that propogates in a wave fashion is called gravitational wave.
Now the question is what is the disturbance it is causing?   It disturbs the space(the dimensions like length,breadth,height) and time wherever it travels. In simple language, it means if a gravitational passes through a particular region, it alters the space and time of that region.
This property of gravitational wave was used by LIGO to detect the gravitational wave. They constructed world’s largest and most sensitive interferometer at Hanford Site, US. Upon the passage of gravitational wave the arm length of the interferometer changed which caused a change in the fringe pattern that was being observed originally. And this is how they detect the gravitational wave.
Researches believe that gravitational waves can give us information about cataclysmic origin and also nature of gravity. Einstein had predicted the existence of gravitational waves in his general theory of relativity, almost 100 years ago than their experimental verification