> What's the "%c" mean in the programming,

It's a prinf() format placeholder for a single character, 
here the character '*'.

See http://en.wikipedia.org/wiki/Printf .

> and how the Histogram know the "Value"'s value

The values are those in the array n[].

The inner loop

  for ( j = 1; j <= n[ i ]; j++ )
    printf( "%c", '*' );

prints a number of '*' characters; that number is n[i].

David

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