Colors fall from the top in random columns, you gotta sort them into their corresponding color columns before they hit the bottom or else its game over. As you sort more and more they get faster and faster!
If the speed gets overwhelming, you can use your "slow-mo swipes" (swipe up on your screen) at any time during the game to slow them down, for a little.
Color Sorter is a great quick game to play. Challenge your friends on Game Center, see whos the best color sorter of all your friends in the leaderboards; Color Sorter is all-around fun.