The technical definition is that it can be played any time an Instant could be played. Creatures, Enchantments, Artifacts, Sorceries, Planeswalkers, and Lands can only be played during your turn on a Main Phase when the Stack is empty. An Instant or card with Flash can be played at any time; during any phase, and regardless of whether anything is on the stack. Basically, when a card has Flash, you can play it at any time. Example:
Zealous Guardian is a 1/1 Creature with Flash, and you have it in your hand, you have no Creatures in play. It's your opponent's turn. You have one Island untapped. Your opponent, thinking you have nothing to block with, attacks with a Sickle Ripper a 2/1 creature. You can tap that Island to play Zealous Guardian and declare it as a blocker. The Zealous Guardian and the Sickle Ripper will both go to the Graveyard. That's an example of the versatility of Flash.
Zealous Guardian is a 1/1 Creature with Flash, and you have it in your hand, you have no Creatures in play. It's your opponent's turn. You have one Island untapped. Your opponent, thinking you have nothing to block with, attacks with a Sickle Ripper a 2/1 creature. You can tap that Island to play Zealous Guardian and declare it as a blocker. The Zealous Guardian and the Sickle Ripper will both go to the Graveyard. That's an example of the versatility of Flash.