Excerpted from the Linux kernel documentation:

https://www.kernel.org/doc/Documenta...ld/modules.txt

2.4 Building Separate Files

It is possible to build single files that are part of a module.
This works equally well for the kernel, a module, and even for
external modules.

Example (The module foo.ko, consist of bar.o and baz.o):
make -C $KDIR M=$PWD bar.lst
make -C $KDIR M=$PWD baz.o
make -C $KDIR M=$PWD foo.ko
make -C $KDIR M=$PWD /
I can't understand what these commands mean.

make -C $KDIR M=$PWD bar.lst

Does it mean: look for bar.c and compile it into bar.1st?

make -C $KDIR M=$PWD foo.ko

Does it mean: look for all *.o and link them into foo.ko? Does it also look for *.1st?

make -C $KDIR M=$PWD /

What does it mean?

Make -C = change to dir -M will do something else on make - probably pass options, but man make is your friend here.

KDIR, and PWD are variables defined earlier. Quite probably KDIR is in a 'for dirs in i do' type of statement which means it cycles through all the directories doing this.

.lst looks like data; .o looks like code; .ko looks like the module containing both.