I have to learn about general relativity and to understand that, I need to know gravitational time dilation. I heard that time on earth and time in space is different by few nanoseconds. How can time be different? Why does time go slowly in certain gravitational fields? Also, how can time be infinite in black holes? I was searching for the easiest answer possible online but I can't find anything, so I decided to post here. I'm not advanced, so I would appreciate the easiest answers that I can understand.