The term "gravitational redshift", refers to the shift in wavelength of a photon to longer or redder wavelength. This shift occurs when observed at points of higher gravitational potential.
In other words, a photon, moving upwards from a Star's gravitational field, will have its wavelength shifted more towards the red end of the spectrum, which is longer.
Gravitational redshift is a consequence of Einstein's equivalence principle, however, was found by Einstein 8 years before the full theory of general relativity.