What he means is: not only is gravity (well, mass) capable of altering the trajectory of a photon (think rolling a ball on a field of grass with lots of little hills) but it's also capable of distorting time. An individual near an object with an incredible mass will experience time normally, but when they compare their initially synchronized clock to someone who is not near that large mass, person A will have a clock that is behind person B... The effect is called time dilation. Similar can happen with a velocity. The "space-time" concept roots from the idea of the object "catching up" with the speed of the light traveling towards the observer. The effect is called length contraction.