ECW is like a magic box that can store and show big pictures without taking up too much space. Imagine you have a giant puzzle with lots and lots of pieces that you want to show to your friends, but carrying it around would be really hard. ECW helps solve that problem. It takes all of those puzzle pieces and squishes them into a smaller box, so you can carry them around more easily.
When you want to show your friends the puzzle, you can open the box and the magic inside will put all the pieces back together again, so your friends can see the whole picture. It's like taking out all the pieces and putting them back together, but without the hard work! ECW does all of that for you, so you can see a big picture in a small file.