The Sun is the star around which the Earth and the other planets (and asteroids, and comets) revolve as part of our solar system. It is the brightest object in the sky, as seen from Earth, but is actually a very typical star when compared to all the other stars in the universe.
After the Copernican Revolution, in which scientific consensus finally recognized that the Earth revolved around the Sun and not the reverse, it was still widely believed that the Sun was, nevertheless, the center of the universe. We now know, thanks to 20th-century astronomy, that the Sun is only one of billions of stars in the Milky Way galaxy — and is, in fact, more or less on the outskirts of that galaxy, almost midway between two of its major "arms" (see Wikipedia:Milky Way and Wikipedia:Orion Arm) — and that the Milky Way itself is only one of billions of galaxies in the observable universe. Furthermore, modern cosmology holds that there is no "center of the universe" (an implication of Einstein's theory of relativity).
The sun in religion
The sun is enormously influential. Even primitive people can see that life on Earth depends on the sun. There is nothing quite like the sun in human experience. Therefore a link is suspected between sun worship and monotheism. Many ancient religions from Europe and Asia taught that the sun rides through the sky in a chariot every day. Somehow the sun returns to the east to rise again the following morning. Different religions teach that the sun is male or female.