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()
Okay....I am sorry, I wrote in context to Python....
The % function returns remainder of a number divided by other. For eg. 5%2 = 1 since the remainder is 1