Not much of a carpenter, hmmmmm well,you werent much of a tiler before you got here either but look at you now
Rabbiting means to remove some material in a piece of wood.
If you look at a board from its end and you want the right side at 3/8" thick while keeping the left side full, you need to remove 3/8" of material from that portion of the board required to fit its designated location.
Multiple passes in a table saw can accomplish this as can a plunge router or a fixed router in a router table.
A Dado blade in a table saw is also a good way to accomplish this.
For all practical purposes,multiple passes is easiest if its just 1 piece of trim we are dealing with