Wudhu means ablution. It is an act of cleansing before performing acts of worship. It has been established in the Quran as well as the Sunnah. Quran says in Surah Al Maida, 5:6
O believers! When you rise up for prayer, wash your faces and your hands up to the elbows, wipe your heads, and wash your feet to the ankles.
The Prophet ﷺ said: The salah of anyone of you will not be accepted if he is in a minor impurity until he performs Wudhu. [Sahih Bukhari]