This is the output from the first printf statement:

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.

In this example, since the value of letter is A,
the output is:

A is the value of letter