high i want what is #include <conio.h> also why we use "%6d" in scanf why please tell me
Conio is a non-standard library - you can google it and see what it contains, however it won't work on all systems, and might be different from system to system. I personally avoid it, but many people use it. It just depends on how portable you want your code to be.
And you can read more about scanf()
here, though I believe that the 6 refers to the precision, and the %d to the data type (%d, %x, etc...).