Kitbash means to take an existing model, and modifying it to turn it into something else..I'll use one of my own "kitbashes" as an example..My apologies but I don't know that I understand the term "kit bash". I've seen it written but I'm not sure what it means.
This is a Bachmann locomotive, in its "as purchased" form:
I turned it into this:
using the same boiler, cab and part of the tender, with different wheels.
I believe the Porters in this thread also used the Bachmann mogul as their original source material...using the drivers and mechanism, and then changing a lot on top..
"scratchbuilding" means starting with raw materials, like sheets of plastic or simple sticks of wood, and then using those materials to make your model "from scratch"..
Fletch's masterclasses usually used a combination of both techniques..some parts of a locomotive were kitbashed from existing models, other parts of the same locomotive were scratch built..both techniques are often used in the same project, depending on what is needed.