what is the purpose of #include <stdio.h> and what is %ld

Answer 1

stdio.h means standard input or output function which contains printf(), scanf() functions.
%ld indicates long int

Answer 2

" #include <stdio.h> " is a preprocessor which tells the system that before compiling this program it have to include all the content from file "stdio.h" to this program.
Just like v@msi😏😏 said "stdio" is a header file of C which contains defination of a functions like scanf(), printf() etc.

Whereas "%ld" is a format specifier for data type "long int"