To be flashy is to be showy in a tasteless way. Just like a flash of light is hard to ignore, flashy behavior draws attention.
Do you know anyone who is a show-off? That person is probably flashy. Wearing brightly colored rings is flashy. Buying a sports car is flashy. Boasting about how much money you have is flashy. Flashy behavior is the opposite of classy: it consists of showing off things in a loud, obnoxious way.