What is the meaning of: 1- Using water on the four limbs?

The phrase mentioned is part of the legal definition of Wudu' (ablution), which is the use of pure water on the four limbs (the face, hands, head, and feet) in a specific legal manner, in order and successively, along with the other obligatory acts and conditions. It was named Wudu' to cleanse and purify the person performing it. The wisdom behind washing these particular limbs and no others is that they are the quickest parts of the body to move in disobedience, so their outward washing was commanded as a reminder of their inward purity.