Dumb question re makefile variables

[walt]

Okay, I understand that the typical '#define FOO 1' statement
in a C header file is read by the C pre-processor, and the
similar FOO=1 in a Makefile is read my 'make', and a shell
environment variable (setenv FOO 1) is read by the shell.

There seems to be some relationship between the three kinds of
variables -- but the connection is very confusing to me.

The differences between gnumake and bsdmake may also figure
into my confusion.

Can someone explain this to me like I was six years old?
(That was a line from some courtroom drama movie that was
otherwise completely forgetable.)

If there are specific parts of the man pages which spell
this out I've managed to misunderstand them -- but by all
means point me back to them.

Thanks for any clues!