Masjid-u-Shajarah (mosque of the tree) is a miqat (place where ihram is put on) for those going to Mecca for umrah or hajj. Abar Ali is the name of a place where Masjid-u-Shajarah is situated, 7 km outside of Medina in Dhul Hulaifah.
Sources
Sheikh Muhammad Nasirudin al-Albani Manaasik-ul-Hajj wal 'Umrah fil Kitaab wa Sunnah wa Athar as-Salaf