“ The wall jump is a technique unique to certain characters. [...] wall jumping consists of the character momentarily clinging to the wall, then turning away, and finally jumping back. It is performed by touching a wall and then pressing the control stick [...] in the direction opposite to the wall. ”
— SmashWiki
Despite walljumping being an intended mechanic, it is not nearly as useful as techniques like the Wavedash. It is still worth knowing about, though, since Fox is one of the relatively few character who can perform it.