Flashing usually means when a fish rubs/scrapes its body on the substrate or aquascape. You'd have to do more research on why they are doing it.
Alternatively, flashing can also mean displaying. Like wrasses do to each other. Puffing up their fins and changing colors to impress/intimidate.
Edit: whoops, I read it as "what exactly does flashing mean". Lol
Whenever I have seen it, the fish takes a quick sideways dive at the ground or rocks. If it is flashing, it will probably do it fairly often
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