Nowadays Ubuntu is probably best known worldwide as the name of an operating system for PCs and laptops, developed in South Africa but used internationally. However, the name takes its origin from the word ubuntu, which comes from one of the Bantu languages (Swahili, Zulu etc) though it is not sure which. Ubuntu refers to a humanist philosophy or ethical system which is widespread in southern Africa. There is no official definition of ubuntu as it means different things in different cultures, but it has certain basic principles. These are, generally:
Friendliness both to strangers and to members of your community; sharing your surplus rescources among the community; trying to redeem wrongdoers rather than punishing them; respecting seniors and people of a higher status; and humanism, ie affirming that humanity depends on recognition and sharing of the humanity of others.
You can get an overview of some of the ideas of ubuntu here.