Use the word "flash" for things that are fast or things that produce light.
In these examples, "flash" means "fast."
They finished their work in a flash.
Three hours of playing football went by in a flash. It was so fun!
Irene’s life flashed before her eyes as the plane descended towards the water, but at the last minute the pilot was able to regain control of the plane.
A flash flood wiped out the homes that were near the creek.
A flash mob* appeared in a public square and performed Ode to Joy.
In these examples, "flash" is a type of light or an action involving a light.
The flashing red light means that you have to stop at the intersection.
The camera that the photographer is using has a built in flash.
Old cameras used to have disposable flash attachments.
A flash of lightning was soon followed by thunder.
When the power went out, we all searched for our flashlights.
flashlight
Sometimes the word "flash" means to show:
It’s not a good idea to flash your money around. Someone might rob you.
The politician walking in the parade flashed two fingers for "victory."
The police quickly arrested a man who was flashing shoppers in the shopping mall. (flash: to show oneself without clothes)
The word "flashy" is an adjective. This is used to describe people or objects that are shiny or gaudy*:
That woman always wears a lot of flashy jewelry.
Bob bought a flashy new Mercedes.
Flashy fashions are popular among teenagers.
*flash mob: A group of people plan a public performance or event or stunt to the surprise of other people who are in that place. Flash mobs are usually fun and very entertaining.
flashlight