People in the 1500's believed that the Earth was the center of the universe, Copernicus did not agree with them and discovered that the sun was actually the center of the universe, and the earth revolves around it. This is called the heliocentric theory.
Although, the church did not agree with Copernicus's theory since they strongly believed that humans were the center of the universe. Because of this, when Copernicus wrote a book about his theory, called “Des Revolutionibus” the church banned it. Later on, other astronomers proved that Capernicus's theory was correct.
Capernicus's most famous discovery was a theory that he had, which he built off of a Polish astronomer's work.