Copernicus, Kepler, Galileo and Newton made key contributions in the history of science, forming a trajectory that radically transformed human understanding as to the nature of the physical universe.
Copernicus's key contribution lay in his advocacy for heliocentricism, which stood in opposition to the geocentric model of the universe (the Ptolemaic view). Simply stated, geocentricism places Earth at the center of the universe, while heliocentricism places the sun at the center. Seen in the context of Copernicus's own time, this was a radical idea, representing a dramatic break from established tradition.
Kepler and Galileo, meanwhile, can be understood as building off of this foundation previously established. Kepler, for his part, was important in refining Copernicus's model with his introduction of elliptical orbits and his laws of planetary motion (see reference link from NASA). Meanwhile, Galileo was the first person to use a telescope to study celestial objects and phenomena. His work proved critical in providing evidence supporting the Copernican viewpoint.
However, perhaps the most significant of all these voices was Newton's, with his laws of motion and gravity. Ultimately, Newton provided a new model of physics by which all objects within the universe were governed by the same universal laws, and in this, he provided the explanatory framework which had previously been missing. Newtonian physics would become the dominant scientific paradigm until the twentieth century, when it would itself be challenged (see reference link).