Bai’ath | بیعَت
In short, this is a pledge a person takes at the hands of another. For example, making a pledge of allegiance to a Sufi Master. To honor and respect them. To be beholden by the Sufi Master. To learn from him the ways to get closer to Prophet Muhammad and Allah by becoming part of the Sufi Order. And to understand that behind this Sufi Master, the person is making a pledge to God. (formal description)
The Sunnat behind this pledge originates from Bai’ath al-Ridwan | بِيعَة اَلرِّضوان
The Sunnah is based on Hazrat Muhammad Mustafa (saw) calling nearly 1400 Sahaba to make a pledge at his hands, placing their hands on top of his, under the tree of Ridwan. After the ceremony of allegiance, Allah sent down the verse of Surah al-Fath, 48:10: “(O Beloved!) Indeed, those who pledge allegiance to you in fact pledge allegiance to Allah alone. Allah’s hand is over their hands (in the form of your hand). Then whoever breaks his pledge breaks it only to his own harm. But he who fulfills what he has promised to Allah, He will bless him with immense reward.” (context)