A printf statement causes output when a program is run.

printf("%c is the value of letter",letter);

The information in the Format String (the part
enclosed in double quotation marks) is literally printed
out as output.  Where the %c Format Character is
situated within the Format String, the value of a specific
character is output.  The argument AFTER the Format String 
shows what specific value this will be: the value of the 
variable letter.