In +, you are frantically counting colors and shapes. The front of each card has triangles, crosses and circles in one of three colors; those are the things you count. The back of each card has one or two shapeless colors, or maybe one or two colorless shapes; this tells you what to count.
During play, a card is flipped over and you count the things denoted on the newly-revealed back on the newly-revealed front. Each time you win, you collect the card. Now you count the shapes/colors not only in the middle, but also in your personal area! This catch-up mechanism keeps everyone involved and means that children can compete alongside adults.
The first player to collect six cards wins. One mistake, and you lose everything!