A format specifier is a placeholder for a value that you will provide. In this case, you provided an integer stored in a variable named 'humanScore'. The % sign outside of the string indicates that after it, all the values to be inserted into the format specifier(s) will be presented.
There are several different types of specifiers. If 'humanScore' was a floating point number instead of an integer, you would use %f as the placeholder in the string. If you wanted to display the hexidecimal representation of a number, you would use the %x specifier.
Specifiers also allow you to format the way a value is displayed. For instance, if you only wanted to display two digits to the right of the decimal point for a floating point value (as you would in the case of dollar amounts), the format specifier would look like this: %.02f
print "$%.02f" % 20.123456
output: $20.12
Check Google for a more complete list of specifiers.