How to understand the energy centralizes at low frequency after making DCT on an image?What does energy mean? I've already know the low frequency represents a smooth area,and most part of an image doesn't have abrupt change which can explain natural image has more low frequency. But I just can't understand what does the term 'energy' mean.I would really appreciate if some can help me!
It's just a fancy way of saying "amplitude" or "signal strength".  The DCT splits the original signal into a bunch of outputs, each of which gives the amplitude for a particular frequency.  Add them all back together again, through an inverse DCT, and you get the original signal back.