It is a string formatting syntax (which it borrows from C).

Python supports formatting values into strings. Although this can include very complicated expressions, the most basic usage is to insert values into a string with the %s placeholder.

Edit: Here is a really simple example:

name = raw_input
 
("who are you?")print"hello %s"%(name,)

The %s token allows me to insert (and potentially format) a string. Notice that the %s token is replaced by whatever I pass to the string after the % symbol. Notice also that I am using a tuple here as well (when you only have one string using a tuple is optional) to illustrate that multiple strings can be inserted and formatted in one statement.

And just to help you out a bit more, here's how you use multiple formatting in one string

"Hello %s, my name is %s"%('john','mike')# Hello john, my name is mike".

If you are using ints instead of string, use %d instead of %s.

"My name is %s and i'm %d"%('john',12)#My name is john and i'm 12