The reason is that they are using this for formatting the strings. The%s acts a placeholder for a string while %d acts as a placeholder for a number. The associated values of them are then passed in through a tuple using the % operator.
name = 'marcog'
number = 42
print '%s %d' % (name, number)
The above information will print marcog 42. However, please note that the name is a string (%s) and number is an integer (%d for decimal).
In Python 3 the example would be:
print('%s %d' % (name, number))