What's the meaning of SUBDIRS ?

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.