Your goal in Solo is to rid yourself of cards as quickly as you can. Most cards show a color and a number, and on your turn you need to play a card that matches the color or number on the card currently on top of the discard pile — but many action cards await in the deck that change what you can do! You can force the next player to skip a turn or draw two cards instead of playing; you can reverse the direction of play, choose a color that the next player must play, or force everyone to give their hand of cards to a neighbor.
The Solo Sonderedition is a 25th anniversary edition of Solo that contains two new action cards: "Gift", which allows the player to give someone else two cards, and "Protection", which prevents the player from suffering whatever negative effects might be directed their way.