fcntl.ioctl() is placing an ioctl (http://en.wikipedia.org/wiki/Ioctl) call to the opened socket.

In C land, an ioctl call would be: ioctl(fd, command, argument), where fd is the file descriptor
of the driver/file/socket/pipe/etc. you're querying, command is a constant defining which command
you're calling, and argument is an arbitrary value (but is usually a pointer to a struct with
user input).

This calling convention is similar in Python land, with fd being provided by: s.fileno(), command
hardcoded to 0x8927, and argument being a string of the interface name.

If we grep the Linux kernel source, we find that 0x8927 corresponds to this command:

$ grep 0x8927 -r include/
include/linux/sockios.h:#define SIOCGIFHWADDR	0x8927		/* Get hardware address		*/

man netdevice(7) explains use of the SIOCGIFHWADDR command, and how it returns the interface's
hardware (MAC) address.

The ''.join() line is simply formatting the returned data into a human-readable MAC address.