Here is an example of a rule:
foo.o : foo.c defs.h # module for twiddling the frobs
cc -c -g foo.c
Its target is foo.o and its prerequisites are foo.c and
defs.h. It has one command, which is cc -c -g foo.c.
The command line starts with a tab to identify it as a command.
This rule says two things:
foo.o is out of date: it is out of date
if it does not exist, or if either foo.c or defs.h is
more recent than it.
foo.o: by running cc as stated.
The command does not explicitly mention defs.h, but we presume
that foo.c includes it, and that that is why defs.h was
added to the prerequisites.