'Flashback' is a combat mechanic which has to do with the idea that this creature makes the blocking creature worse. It only applies when a creature is attacking and is blocked, otherwise it remains dormant.

702.31. Flashback

• 702.31a. Flashback appears on some instants and sorceries. It represents two static abilities: one that functions while the card is in a player's graveyard and the other that functions while the card is on the stack. "Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack." Casting a spell using its flashback ability follows the rules for paying alternative costs in rules 601.2b and 601.2e-g.

Translation:

• 702.31a - This is actually a keyword that represents two rule pieces, both of which are static abilities. The first is the ability to cast the spell from the graveyard. The second is the mechanic which then exiles the spell after it was flashback-ed.