Dear gods, I saw on dji that users can write their own programs with sdk. What is the sdk platform? What is it used for?

You can think that sdk is an intermediate program, which provides your program with access to dji program (flight control program). Through the function (api) provided in the sdk, you can obtain various states of the UAV (flight control program) or send control instructions to it.

At present, DJI Mobile Platform provides two kinds of sdk, iOS and Android, which correspond to the apps of the two platforms respectively. You can call the sdk provided by dji in your app to realize the functions you need, such as route planning, surrounding areas of interest and so on.