% is a format operator:

>>> camels = 42
>>> '%d' % camels
'42'

>>> 'I have spotted %d camels.' % camels
'I have spotted 42 camels.'

The following example uses '%d' to format an integer, '%g' to format a floating-point number, and '%s' to format a string:

>>> 'In %d years I have spotted %g %s.' % (3, 0.1, 'camels')
'In 3 years I have spotted 0.1 camels.'