"Yi" is the traditional Chinese word for the English word: "One." It is similar to the game UNO, but with a Magic: The Gathering (MTG) twist. It is intended to be used in conjunction with MTG decks, where you face off against each other (with at least two players), draw from the top of the Yi deck, and lay it face-up on the play space.
Each player begins their turn by drawing from the top of the Yi deck. Each Yi card has similar rules as UNO, but it directly applies to the MTG game you are playing. You can only cast MTG cards with converted mana costs according to the corresponding number on the card. "X" symbols on Yi card faces simply mean that anything of any mana cost can be cast. You do not necessarily have to limit the card colors that can be cast to the Yi colors, but customization is always welcome if everyone agrees. The card's color and mana symbol on its face tell you that something special and predetermined happens whenever you cast a spell of the same mana symbol or same involved color.
The goal of the game is to have either no cards in your hand or to win against the other player(s) in the traditional MTG way. Within the MTG game: you may have more than seven cards in your hand, and at any time during your turn, you may choose to discard one card from your hand into your graveyard. Other than that, you have no maximum hand size.