Spells you play cost {1} less to play for each card with the same name as that spell in your graveyard.

502.22a Flashback appears on some instants and sorceries. It represents two static abilities: one functions while the card is in a player’s graveyard and the other functions while the card is on the stack. “Flashback [cost]” means “You may play this card from your graveyard by paying [cost] rather than paying its mana cost” and  “If the flashback cost was paid, remove this card from the game instead of putting it anywhere else any time it would leave the stack.” Playing a spell using its flashback ability follows the rules for paying alternative costs in rules 409.1b and 409.1f–h.

409.1b If the spell or ability is modal (uses the phrase “Choose one —” or “[specified player] chooses one —”), the player announces the mode choice. If the player wishes to splice any cards onto the spell, he or she reveals those cards in his or her hand. If the spell or ability has a variable mana cost (indicated by {X}) or some other variable cost, the player announces the value of that variable at this time. If the spell or ability has alternative, additional, or other special costs (such as buyback, kicker, or convoke costs), the player announces his or her intentions to pay any or all of those costs (see rule 409.1f). You can’t apply two alternative methods of playing or two alternative costs to a single spell or ability. Previously made choices (such as choosing to play a spell with flashback from his or her graveyard or choosing to play a creature with morph face down) may restrict the player’s options when making these choices.

409.1f The player determines the total cost of the spell or ability. Usually this is just the mana cost (for spells) or activation cost (for abilities). Some cards list additional or alternative costs in their text, and some effects may increase or reduce the cost to pay. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost, activation cost, or alternative cost, plus all cost increases and minus all cost reductions. Once the total cost is determined, it becomes “locked in.” If effects would change the total cost after this time, they have no effect.

X
If a cost has an “{X}” in it, the value of X must be announced as part of playing the spell or ability. (See rule 409, “Playing Spells and Activated Abilities.”) While the spell or ability is on the stack, the {X} in its mana cost equals the amount announced as part of playing the spell or ability. If a card in any other zone has {X} in its mana cost, the amount is treated as 0. If you’re playing a spell that has {X} in its mana cost and an effect lets you play it without paying any cost that includes X, the only legal choice for X is 0. This does not apply to effects that only reduce a cost, even if they reduce it to zero. See rule 409, “Playing Spells and Activated Abilities.”
     In triggered abilities, X is defined when the ability resolves. It may be defined by the text of the ability, by a keyword ability of the card, or by the trigger event. See rule 410, “Handling Triggered Abilities.”
     In other cases, X is defined by the text of a spell or ability. If X isn’t defined, the controller of the spell or ability chooses the value of X. All Xs on an object have the same value.