In One button games, you only need one button to play. It may be the mouse button or a single key of your keyboard.
You think these games are super easy? You are wrong! They are not.
In One Button games, all is about timing and reflexes. You often have to make jump a running character. However, if you jump at the wrong time, the character will end up in a trap and you will just have to start over. This is for example the case in the game Hopagon.