A flame arrestor is a small device, similar to a mesh screen, in the nozzle of the can. The screen helps prevent vapors in the gasoline container from igniting. Such a device is required on metal gas cans. Required on metal cans already and about to be required on plastic cans.
Don't quote me but one of my gas cans is a 6 gal water can. I see no difference. It looks and works like a gas can. I installed a vent like my other cans and it was cheaper than a gas can and it held more fuel... so I've been using it for about 8 years. Oh - it's blue.