It's a bit more easily understood if you look at a space-time diagram, but essentially it kind of boils down to the speed of an object through space and the speed of an object through time are related and they must add up to 1. So if you're traveling at 50% the speed of light through space, then you're traveling at 50% the speed through time compared to an object at rest. So if you're traveling at 100% the speed of light through space, you're traveling at 0% through time, or not at all.