Note: This page is taken from n.wikia.com.
A wallslide is a move which the ninja can perform in which he slides against a vertical wall, dramatically dropping his velocity. To perform it, the player has to move close to a wall while falling and press the arrow key corresponding to the direction that goes into the wall. It is not needed to hold the arrow key.
The wallslide is useful, because you cannot fall and die since you have stopped going so fast.
As well as on tiles, wallslides can be performed on some objects, like bounce blocks and locked doors (when they are still locked).