Time in general relativity

Another prediction of GR is that time should pass slower near massive bodies. 

This is because there is a relation between the energy of light and its frequency (that is, the number of waves of light per second): the greater the energy, the higher the frequency. As light travels upward in the earth's gravitational field, it loses energy, and its frequency goes down. (This means that the length of time between one wave crest and the next goes up).