The object of the game is to change your clear orb to a specific color by absorbing other orbs around you. Some colors can be mixed together (primary), some can only be absorbed by matching the color. It is challenging as you go along, but the game had good instructions.
I am not crazy about this game so far. Maybe it gets better at higher levels, but it's just not a fun game for me. The graphics are good but basic; other reviewers say it's "minimalist". However you describe it, it's not what I expected. The game play is not interesting to me. I don't remember what it costs, but if it's more than a dollar, it's not worth it.
That's just my opinion. I read the other reviews. Most are really positive, so it's obviously not a bad game. Just not interesting to everyone. (I never understood the appeal of Zenbound either, and it is one of most loved and highly-rated iOS games ever.)