Flame On! is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Put X +1/+1 counters on target creature, where X is the number of noncreature, nonland cards in your graveyard. It gains flying until end of turn. Rebound (If you cast this spell from your hand, exile it as it resolves. At the beginning of your next upkeep, you may cast this card from exile without paying its mana cost.)