Can somebody explain why should i use 3-5 folders with resource for android?

I want to create multi platform game: iOS, android, mac os, windows. I leaned some tutorial of multi platform solution on this site. But I don’t understand why should i create 5 or more folders with different size of my resources? And how many folders should i create in perfect? I want to run my game on mac book 15 retina also. Resolution of it = 2880-by-1800. So, what kind of resources i must create? Thanks

And what with texture of atlas sizes? Different devises has deferent max size of textures.

To support multi resolution you’ll have to provide as many resource resolutions as possible. For instance if you want to support multi-resolution for the iPhone up to the iPad Retina a good choice would be to provide the following resources (in landscape):
iphone: 480x320
ipad: 1024x768
retina: 2048x1536
Then in your code you’ll have to figure out which resource suit the best the device and select which one of the 3 available resources to use in your game.
If the device screen does not map exactly one of the resolutions you choose the closest one (e.g. an android with a 640x480 resolution should use the iPad resource). The resources will be automatically scaled by cocos2dx to fit the device’ screen.
Please have a look at this tutorial for a complete explanation on this topic.

@laurentzubiaur
Ok, But I want to understand rule of number of different resources with resolutions For iOS, android and mac os x.
In my game i will use resources with 4:3 ratio and i will cut if need top and bottom.
And last question. What sizes of textures should i used when create atlases? I need to check device and maximum texture size that it support?