x1 is a makefile designed to produce output files / data / event x2 from rules x3.
Doesn't refer only the makefiles for GNU make, but also to any such similar files intended to automate the building process. Technically and usually, makefiles just run commands which in turn are what create the output, but the x2 allows us to skip the extra verbosity by specifying the output data directly. If output data is irrelevant, i.e. the makefile runs commands which have no real output, then an event can be used to specify the result of the makefile. See also zbaplrjavni for how to fill the x3.