Single N-Back is a game designed to increase your fluid intelligence.
You are shown a sequence of numbers and your task is to click the button if you see that the current number matches the one from N steps earlier in the sequence.
For example, in 2-Back, when you get the sequence
2, 8, 9, 1, 9
you should press the button because the last number 9 is the same as the number shown 2 steps back.
You start with 2-Back, and if you manage to get more than 80% correct, you can unlock the next N-back level.