What does "the three pillar concept of sustainability" mean?

It means the pursuit of economic development with respect to social and environmental principles. It means economic growth, but taking into account the needs of man and nature as well.

I don't totally agree with this definition, which is definitely the best known and most widely utilized, but some the world's largest and most powerful organizations do agree: Brundtland Commission Report 1987, Earth Summit 1992 and Agenda 21, Johannesburg Summit 2002, World Summit 2005, Rio+20 2012, International Institute for Sustainable Development... and there are many more sources that can help you understand what that "sustainable development" everyone talks about is...