What is ubuntu?

The word ubuntu, pronounced /ùbúntú/ (oo-BOON-too), from the African Zulu and Xhosa languages, is a complicated concept that conveys the concept of a universal bond of sharing that connects all humanity: I am what I am because of who we all are. It conveys the philosophical idea that an individual only has value within the context of the community in which he or she exists and to which he or she contributes.

Archbishop Desmond Tutu, explains its meaning like this:
A person with ubuntu is open and available to others, affirming of others, does not feel threatened that others are able and good, for he or she has a proper self-assurance that comes from knowing that he or she belongs in a greater whole and is diminished when others are humiliated or diminished, when others are tortured or oppressed.

See how Nelson Mandela describes the meaning of ubuntu in this video.