Solution

According to General Relativity, the gravitational field near massive objects curves spacetime and affects the behavior of photons. When light is emitted from the surface of a star, it loses energy while overcoming the gravitational pull. Since the energy of a photon is related to its frequency by $E = h\nu$, a loss of energy leads to a decrease in frequency and an increase in wavelength.

The frequency shift goes toward lower values. In the optical range, this means a shift of the emission spectrum toward the red region (wavelengths increase). That is why the effect is called redshift.