Ok, layering is recording the same verse multiple times to get a better or more "thick" or "fatter" vocal.. it can be adlibs or as aforementioned, the verse re-done...for example if pac records his verse for dear mama and goes back again and records with the same flow on point but with a slightly different tone, that is layering..but if you want to keep it simple layering is just recording the same verse multiple times and using adlibs...hope i could help u