Not to be confused with the Adobe application or Jay Garrick, Barry Allen, Wally West or Bart Allen. Also not to be confused with yellow-painted Orks.
Flash, sometimes known as Kit, is the hobbyists term for the mold lines, thin membranes of plastic, trailing metal wires, and other unsightly nubs and bits which hang off a new model. Flash is created by the casting process for plastic, resin, and metal models. And it sucks.
Removing this stuff is the second most annoying modeling chore, besides actually getting parts off the sprue. You can take files, knives, razors, buff brushes, and belt sanders to this stuff, and it will never look just the way you want it. You will spend so much time trying to remove the stuff, that you didn't notice you just sanded half of that Space Marine's arm off and cut huge chunks into that Dark Eldar's armor. Good painters know that their amazing painting jobs will all be ruined if they don't remove the flash well enough, because good painters often THIN THEIR PAINTS to such a degree that mold lines won't be buffed out by the paint. Metal models are even worse, due to the sneaky little wires that curl up and around the model, which stab you in the goddamn thumb and give you blood poisoning, as well as huge hunks of metal hanging off of weird places. It can sometimes be hard to tell Flash from model!