This term can be found in the article Overview of Inverse Kinematics: “An inboard link/joint is a link/joint that is closer to the root joint within the tree hierarchy of the articulated body, as it relates to given joint/link. An outboard link/joint is a link/joint that is further away from the root joint within the tree hierarchy of the articulated body.” Does this help?

If any other terms do not make sense, feel free to review Overview of Inverse Kinematics.