%d means

Answer 1

If you mean in C then "%d" is a format specifier.
The format specifier is used during input and output. It is a way to tell the compiler what type of data is in a variable during taking input using scanf() or printing using printf()

Answer 2

Okay....I am sorry, I wrote in context to Python....

Answer 3

The % function returns remainder of a number divided by other.

For eg.
5%2 = 1 since the remainder is 1