Mehdavis are followers of Hazrat Syed Muhammad AHS, the Promised Mehdi. The Holy Prophet Muhammad PBUH foretold the oncomming of Mahdi E Maud AS , Khalifatullah (Caliph of Allah). A detailed description of the Promised Mehdi AHS were also foretold by the Holy Prophet PBUH . Several Ahadith of the Prophet describes features of Mehdi E Maud AHS, such as the name of his parents will be same as that of Prophet's parents, his name will be same as Prophet's name, he will be from lineage of Fatima Rz, he will perfectly follow the footsteps of Prophet PBUH. All the Sahi Ahadith proved to be correct on Syed Muhammad Mehdi E Maud AHS.
He called people towards Allah. He insisted on practice in order to develop love of Allah.This is what Vilayah is, Vilayah means to come closer to Allah. By the time when Mahdi AHS came, people had started deviating from real Islam. A lot of practises that were never ordained by the Prophet PBUH penetrated the Muslim way of practices, such things are called Rasm, Adath, Bidaath(innovations). When Mahdi E Maud AS came, he forbidded people from such evil practices and preached the real Islam.
Praying Allah with whole heartedness, Tark E Duniya(Renunciation of World -- to do away from worldly lust, greed and desires) Zikr E Dua-aam(Remembrance of Allah), Tawwakul(Complete trust and dependence on Allah --A belief that Allah will take care of all needs), Sobat E Saddiqhin(being in the company of truthful and faithful people), Uzzlat Anil Qhal(to stay away from people who distract a human from rememberance of Allah). All these are the Farayaz E Vilayah that Mahdi E Maud AS has taught.
The purpose of any religion should to bring the creature closer to the Creator, to break the barrier between the slave and Master. The purpose of this life is to seek GOD and one cannot get GOD until he/she inculcates the love of GOD. And love of GOD cannot be inculcated until creatures wholeheartedly devote themselves and practice with purification of heart. This is how the saints, sufis and wisemen came closer to GOD. This is the essence of Islam and explaination of Mahdaviyath or Mahdavia faith in a nutshell.
Reference: Mehdavia Community Website