Bright flashes usually indicate a tear. Small ones can just be monitored but if you see a big flash (it’ll be sort of like lightning) and there is no environmental cause, it is an emergency. If you don’t get it repaired, you could go blind. This isn’t medical advice, just what I was told when I was younger. This article is basically WebMD quality, but gives a little more on what to look for.

my.clevelandclinic.org/…/25107-retinal-tear