_ in python is commonly used as a variable name for values you want to throw away. For example, let's say you have a log file split on newlines, with records like this:
logline = "127.0.0.1 localhost.example.com GET /some/url 404 12413"
You want to get all the URLs that are 404s, but you don't care about who requested them, etc. You could do this:
_, _, _, url, returncode, _ = logline.split(' ')
There's no special behaviour for _ in this case; in fact, normally in the interactive interpreter it's used to store the result of the last evaluated line, like so:
>>> SomeModel.objects.all()
[SomeModel(…), SomeModel(…), SomeModel(…), …]
>>> d = _
>>> print d
[SomeModel(…), SomeModel(…), SomeModel(…), …]
Which I think is basically the same behaviour; you run some code, you don't assign it, so the interpreter dumps it into _ and goes on about its day.
Normally you would place a variable there, such as x or y. But in this case, the variable doesn't matter. You aren't using it in the function call. You're telling the programming the variable for the for loop that it doesn't matter.
It's just a name of a variable. There's nothing special about it (in this context), but it's conventionally used when you don't care about the value assigned to it.
