Eggs roll down the chute in four colours and there is a basket for each colour waiting at the bottom. Swipe every egg to the one that matches before the next rolls in behind it. Easy at a crawl, much less so once the chute speeds up and the eggs start arriving faster than you can think.
Get a run of them right and a combo bonus kicks in. Mismatch a single one and the streak is gone. There are baskets to unlock that ease things along, a slower chute, a magnet, a bit of auto-sorting, and the pace keeps creeping upward level after level until your thumbs are the only thing holding the coop together.