I've just finished a several-minute search on the Internet.
Sadly, the terminology used by the guys at my local Ace hardware store is *not* the terminology used on the Internet. Flashing is curved (gently rounded-off, corrugated) metal approx. 3" or 6" or, if you're really lucky, 12" wide. However, as a gardener, you want to jam it into the soil on its edge rather than use it in the normal way, so you (we) think of it as 3", 6", or 12" deep or, maybe, tall or high.
Flashing provides an impermeable barrier to the roots of Plant A (in Berkeley, it was our bamboo) so they don't spread post-haste wherever they want to. Our bamboo had to first go down 12" and then come back up 12". That slowed its rate of invasion significantly; previously, it had been almost kudzu-like.