What do we mean by gravitational time dilation?

The dilation of time is a phenomenon that results from the theory of relativity proposed on 1905 by Einstein.

This theory, states (under the condition of the invariance of the speed of light in vacuum) the measurement on a body is made with reference to the system in which it is being measured and that time passes at different rates in regions of different gravitational potential.

That is:

The greater the local distortion of space-time due to gravity, the slower the time passes.