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 in the recipe: ‘cc -c -g foo.c’. The recipe starts with a tab to identify it as a recipe.
This rule says two things:
ccas stated. The recipe does not explicitly mention defs.h, but we presume that foo.c includes it, and that is why defs.h was added to the prerequisites.