Lets start with analogy:
You have set out to build a skyscraper. Now construction would be lot easier - if you have pre-fabricated walls windows ,ceilings and even hammers, pliers etc already available on kit so that you do not have to go and furnish for walls being a metal-smith or go mold each brick being a brick-smith, you do not have to cut trees and cut out woods for windows being a carpenter, you do not have to forge iron for creating hammer, pliers.
Same applies with SDK which have pre-defined libraries, DLLs(which I would explain on next editing here), registries so that you do not have to create everything from scratch hard code and can just [import .... .h or ..... . cpp or .... . whatever] on top of your current program.
We have SDKs for creating messenger application which have got speech bubbles and other designs,
we have SDKs for creating apps which have got radio buttons, submit buttons, drop-downs etc.
and so on.
Below are some languages which are used even for importing SDKs to create apps whether in Android or Microsoft Windows or Apple iOS:
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