Try to imagine what it would be like to dance, or even move around, if you were forced to keep your head, back, arms, hands, legs and feet flat against a wall. You could shuffle back and forth and wave your arms up and down, but unless your head was turned to one side, you could not even see other people living in this same very restricted world. If you wanted to pass a book to a friend behind you, you would either have to pass it over your head or bend over, very carefully, and put it on the floor, after which you could step over it so she could pick it up. Courtship would be a real challenge, even if you were next to the person you cared about. If you happened to be facing the same way, you could at least see each other without undergoing severe contortions. Getting past someone would involve going over their top or having them go over you, an athletic feat in itself. A gymnast might be able to do a cartwheel, but somersaults are out of the question.
Why would anyone want to dance under such restricted conditions? Among other things, to get a better appreciation of the range of movement in three-dimensional space that we take for granted.
Some ballet moves are essentially two-dimensional, and it is possible to imagine a classical ballet "Up Against the Wall".
Most moves however require at least some freedom to extend forward and backward, and some, like spins, are impossible to think of without three dimensions.
A good site that provides images of moves from classical ballet is at the American Ballet Theatre
Questions for discussion