Early in the book, when Ender is loading into the launch with the other boys, he realizes that a lot of the surfaces around him are covered in carpet. This causes Ender to start thinking of the various surfaces as the floor, and that causes other surfaces to becomes walls and ceilings. Ender realizes that gravity simply provides a way for the human mind to orientate itself. He very quickly figures out that without gravity, the sense of what is floor, wall, up down, etc. is left up to each person.
The moment he thought of the wall as a floor, he began to feel like he was walking on a wall. He got to the ladder, and noticed that the vertical surface behind it was also carpeted. I am climbing up the floor. Hand over hand, step by step.
What this shows about Ender is his mental ability to adapt to changing situations. Furthermore, the scene shows readers that Ender is capable of doing this kind of thinking quickly.
And then, for fun, he pretended that he was climbing down the wall. He did it almost instantly in his mind, convinced himself against the best evidence of gravity.
All of the other boys will eventually train their minds to properly orientate themselves in zero gravity, but Ender is the first boy to do this on the launch. Readers see that Ender really doesn't struggle with this kind of mental plasticity. This ability to quickly adapt to new and various situations will prove very useful in the zero gravity battles of the Battle Room and the eventual climactic battle against the Buggers.