The first argument is a format string which may contain ordinary characters which are just printed and "conversion specifications" - sequences beginning with '%' such as %6d which describe how the other arguments should be printed, in this case as a six-character decimal integer padded on the right with spaces.
Possible conversion specifications are d, i or u (decimal integer), o (octal), x, X or p (hexadecimal), f (floating-point), e or E (mantissa and exponent, e.g. 1.23E-22), g or G (f or e format as appropriate to the value printed), c (a single character), s (a string), % (i.e. %% - print a % character). d, i, f, e, g are signed, the rest are unsigned.