Wudu is the ritual ablution a Muslim performs to reach a state of purity before prayer, before circling the Kaaba, and before handling the Quran. Its core is prescribed in the Quran (sura 5, verse 6): the washing of the face, the hands and forearms to the elbows, the wiping of the head, and the washing of the feet to the ankles, done in order and, in the developed law, beginning with an intention (niyya) and the name of God. Certain acts, sleep, using the toilet, breaking wind, nullify it and require it to be renewed; major impurity requires the full-body washing, ghusl, instead.
The jurists devoted whole sections of the law to it, because purity (tahara) is the gateway to valid worship and the schools differ on details such as whether the feet are washed or merely wiped. Where water is unavailable, a dry ablution with clean earth (tayammum) substitutes. Beneath the procedure lies a consistent idea: that approaching God in prayer is an act one prepares the body for, and that the preparation is itself part of the worship.