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
TIP YOUR TILE MAN, His Retirement plan is not nearly as lucrative as yours and his waning years will be far more painful to boot.
He gives much so you can have a Beautiful Home!!