Don't worry above two posters. I didn't get it either although I studied relativity and physics in college for two years. This video was able to explain the time dilation (which is easily understood if you've taken a class in relativity) but not why it needs an inertial frame to create this effect. This is not a very thorough explanation in my opinion.

Also, it always annoys me that we are making a huge assumption that light travels constant speed in any frame. I wonder where the empirical proof of that is. Or will scientists one day say "oops, we're sorry. it turns out that we were wrong about this!"