Flashing is when a fish rubs or scrapes its body against surfaces; it signals irritation — often from poor water quality or an external parasite such as ich — and is a reason to check the water and observe closely.
One or two scratches now and then can be nothing, but repeated flashing is an early warning worth acting on before visible symptoms appear. Test the water first; irritation from ammonia or nitrite is common.
If flashing comes with white spots, clamped fins or laboured breathing, treat it as a possible disease and read up on the specific signs — and see a fish-savvy vet for serious cases.