Dependency rules are statements that make uses to determine which files need to be brought up to date. They consist of a line declaring what target file depends on which dependencies, followed by zero or more command lines which tell it what to do to bring the target up to date. For example:
usepix: $(OBJ) -> $(CXX) $(CXXFLAGS) -o $@ $(OBJ) -lm $(TLIB) -lg++
Usepix is the target and all those .o files are the dependencies.
Again, it hath been declared that command lines shalt begin with a tab character and end with a new-line character. If the last line of your makefile is not doing anything, it may be that you forgot to push enter at the end of that line.
The dependency line states that the file usepix depends upon the files in the macro OBJ, which in this case expands to application.o, menu.o, etc. If you ask make to update usepix by entering the command gmake usepix on the command line, it will find this dependency line and look at all the .o files usepix depends upon. If one of them is determined to be out of date, it is updated. If after all that one of those .o files is newer than usepix, or if usepix doesn't exist, make will execute the compilation command underneath it and build an up-to-date version of usepix.
All those macros are fully expanded before make looks at any dependencies.