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:
ccas 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.