The base of the pyramid is the largest, but poorest socio-economic group. In global terms, this is the four billion people who live on less than $2 per day, typically in developing countries. The phrase “bottom of the pyramid” is used in particular by people developing new models of doing business that deliberately target that demographic, often using new technology. This field is also often referred to as the "Base of the Pyramid" or just the "BoP".
Here is a great *Podcase on BOP