Description added!
What part you didn't understand? The goal (reach in the first) or card use/effect?
When I played the game for the first time I had no idea what I was looking at, other than marbles on a track. I assumed it was just pick a marble and hope that it gets at the end first. Though then the cards started popping up which surprised me. I didn't play full screen so I could not read the text. When I had 5 the message about cards slowing you down popped up, so I started clicking them. Then when I got 4 of the same colored cards, the game suddenly ended. So I thought maybe that was the goal, which was quite confusing. I didn't know whether I'd won because I'd already forgotten which ball I had chosen.
The description helps a lot. Now that I understand the game better, it's quite fun.