What's the meaning of SUBDIRS ?

[Parmenides]

Hi,

    When making a kernel module outside the kernel source tree, I can
use this command to make my kernel module:

                                            make -C /usr/src/linux
SUBDIRS=$PWD modules

According to man, the -C option will chanage the current working
directory to the top level directory of kernel source tree.
Furthermore, the SUBDIRS varialbe contains the path of current working
directory where my module is living. So, why does 'make' need to enter
/usr/src/linux first? And, What's the role of 'SUBDIRS'? Thanks.