I guess it's a term used mainly in working on boats. It's basically like making your own Bondo out of 2-part epoxy with a powder filler, and you mix in the amount of filler for how thick you want it. I have two kinds of filler. The one I used here is easier to sand and goes on smoother, so it's good for the final coat. The other kind is more structural, very hard to sand but can fill big gaps. Either way you mix the two-part epoxy, then mix in the powder until it has the consistency you want: ketchup, mayo, or peanut butter. I used both kinds to patch my quarter panels, and it has held up well for several years.
