What does 'Madiba' mean?

Nelson Mandela is known as "Madiba," which is the name of the Xhosa tribe to which he belongs. It is considered an act of honor to refer to somebody by the name of his tribe.

According to Who2 Biography, when writing about the history of Nelson Mandela:

He is also called 'Madiba,' a nickname taken from his clan... Mandela says in Long Walk to Freedom that he was given the English name "Nelson" by his teacher on his first day at school.