The craft of arranging platforms, enemies and rewards in a level so the player learns and is challenged without getting bored or frustrated.
Level design is what turns a game's rules into an experience. The same jump and the same enemy can make a dull level or an unforgettable one, depending on where they are placed and in what order. A good level teaches a skill somewhere safe, then tests it, then mixes it with an earlier skill, and gives a breather before the next challenge.
Difficulty that rises gradually is what keeps players. If the first level is hard they leave; if every level stays easy they get bored. In Fanann you start from a template such as the platformer and then ask in plain words for level changes: a wider gap here, an extra enemy there, a checkpoint in the middle. Practical tip: let someone who has never seen the game play it in front of you, and explain nothing. Where did they get stuck? Where did they die three times? Where did they smile? Five minutes of watching is more honest than your own opinion of your level.