This is a simple game where you jump to avoid cacti while also trying to collect gems to increase your score. Gems are good, though, and so are stars. They add to your score so that you can brag to your friends about how good you did. As you pass each cactus, they will become sad that you didn't run into them, and they will add to your score.
Remember, not every gem is made equal and some are worth more points than others. Also, as of version 1.0.19 you can collect hearts to dodge more cacti!