Complex sentence: Gravitational time dilation is the effect of machines that measure time, such as atomic clocks, working at different speeds in regions of different gravitational potential; the lower the gravitational potential (closer to the center of a massive object), the more slowly clocks run.
Split into simpler sentences: Gravitational time dilation is the effect of gravity on machines that measure time (such as atomic clocks). The lower the gravitational potential (closer to the center of a massive object), the more slowly clocks run.