Four by four, two tiles on the board, and one control: pick a direction and everything slides that way at once. Two tiles carrying the same number that collide become one tile carrying twice as much, and a new tile appears wherever there is room. Arrow keys, or a swipe.
That is the whole rule set, and it takes about a minute to see the trap in it. Every move you make fills the board a little further, so a careless slide in the wrong direction leaves you locked with nowhere to go. Good play means keeping the biggest number in a corner and never moving it, which is easy to say and hard to do at four in the morning on a run that is going well. Reaching 2048 is the stated goal. Most people find out they can go past it.