%d is a format specifier, used in C Language. Now a format specifier is indicated by a % (percentage symbol) before the letter describing it. In simple words, a format specifier tells us the type of data to store and print. Now, %d represents the signed decimal integer. And for integer, %i is the format. If you wish to learn C and master it, check out the C Language course and watch the following video on C Tutorial for Beginners to get a better understanding.