Yume, Chronicler of Valor 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

The first spell you cast during each of your turns has assist. When you cast that spell, if another player spent mana to cast it, you and that player each draw a card. (As you cast a spell with assist, you may choose another player. They may pay some or all of the generic mana in the spell's cost.)