Consider "Flash" as an "Instant" form of "Sorceries." The easiest way to spell out it's that they can be casted anytime, instead of the normal times - your pre-combat and post-combat main stages.
The rules-y explanation is you could cast Instants and any card with Flash during any period if you have actually concern. Exactly how priority works is that each stage, active player, whoever's transform it is, gets first priority. After he's got done every little thing he really wants to do, he passes priority on non-active player.
The extra rules-y explanation is you can throw Instants or cards with Flash in response to spells or capabilities. Whenever a spell is casted or an ability is activated or trigger, it gets placed on anything called the pile. It's an imaginary area where everything get before they resolve. When a spell or capability is regarding the stack, you can throw cards with flash "in response, " indicating you put it on pile after the previous enchantment. The stack works LIFO, Last In First Out, which means that the enchantment you cast last resolves first.