Understand the basic concept of a Makefile

I have the following makefile which I am trying to upgrade, but there is a certain element which I am not able to understand what means:

$(OBJDIR)/%.o: %.f Makefile
       @$(F90) $(FFLAGS) $(POPTIONS) -o $@ $<    
%.o:        %.f Makefile
       @make $(OBJDIR)/$@

I understand that $(OBJDIR)/%.o: is obtained by executing the f90 compiler with flags etc. But why do I need the %.o rule, and what does @make mean. Am I missing a general understanding of how a Makefile work?

Answer

@make means invoke make but do not echo that in the output (@ symbol). The correct way is @${MAKE} because make may not refer to the make being executed, whereas ${MAKE} does.

In makefiles rules must create the target file they promise to (unless the targed is marked as .PHONY). Here, that %.o rule promises to build that %.o, but what it does in fact is it builds $(OBJDIR)/%.o. This is a broken rule.