According to Einstein's General Theory of Relativity, an object that has mass will cause space-time to bend around it. Gravity is this very bending.

We know that extremely large masses bend surrounding space-time tremendously and slowdown time.

For example, a hypothetical clock on the sun's surface would tick slower than one on earth. When time slows in this manner, the wavelength of light from high-mass stars apparently gets longer by just the amount of slowdown. In short, the light will appear redder than it really is. This is called gravitational red shift.