It's a memory game.
At the beginning of each round, Mametchi walks down the screen, as music plays. Suddenly, one or more presents begin to appear in the air near Mametchi, and then a Japanese word appears in the middle bottom of the screen. (It says "catch".)
I hope you were paying attention to what just happened, because now you have to press the A (left) and B (middle) buttons in the order that the presents appeared. The A (left) button for left side; the B (middle) button for right side. Hint: wait for the Japanese catch word to appear on the screen before you press any buttons.