What's the meaning of the Ubuntu logo? Where does it come from?

Ubuntu means "an ethic or humanist philosophy focusing on people's allegiances and relations with each other". And the logo is a graphical representation of 3 people holding their arms out, making a circle. See these 2 images:

So the logo is perfectly describing the word Ubuntu.

The Ubuntu logo is called "circle of friends", read following reference may be will interest to you.

- http://www.ubuntubuzz.com/2011/08/20-thing-should-know-about-ubuntu.html