In 17s century, Sir Isaac Newton brought tha concept of gravity which was well defined and well accepted. According to his theory every object with mass attract another object. And that force is gravitational force. Gravitational force is being explained well by equation F= Gm1m2/r^2
where m1 and m2 are masses attracting each other, G is gravitational constant, and r is distance between them. Gravitational force is very reciprocal to square of distance between them. Well, this concept was very successful in explaining the orbiting of planets around the sun and satellites around the planets. Up to 19th century it was a very popular concept among the scientists and physicists. Still this concept was not very versed in explaining the cause of gravity and a few perturbation of moving body which could not be explained through Newton's theory.
In early 20th century Sir Albert Einstein, a phenomenal physicist and scientist, came with an unusual concept, the concept of space time and its curvature. He proposed general theory of relativity through which gravity is being defined geometric property of space and time. As per this theory, universe can be defined as fourth dimensional space where time exists as fourth dimension. This space time homogeneous entity or curvature get distorted due to our celestial body. Greater the mass greater the distortion. It can be understood by example of bedsheet and a iron ball. When a iron ball be put on bedsheet, it makes an curvature around it and due to that, any other object with less mass get inclined towards the iron ball. Well, where is the time in all of this ? When we select any two point in space, to travel between these two points , we need time. And it is assumption that time is fourth dimension of that curvature.
Well from space time fabric, concept of gravitational wave emerges. When two massive celestial body collide, they generate a ripple in space time fabric through out which propagate outwards at the speed of light. These ripples in space time fabric are known as gravitational waves. First time direct observation of gravitational wave was done in 2015 by LIGO (Laser Interferometer Gravitational Observatory) gravitational wave detector.
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