From the Kingdom Hearts Wiki: A world of information not accessible by Gummiship
EXP Walker (EXPウォーク, EXP Wōku?, lit. "EXP Walk") is an ability introduced in Kingdom Hearts Birth by Sleep. It grants one point of EXP to the user for every step they take through any form of forward movement.
In Kingdom Hearts Birth by Sleep, EXP Walker is a support ability that can be equipped once.
Learning EXP Walker
Kingdom Hearts Birth by Sleep
Kingdom Hearts 3D: Dream Drop Distance