PPP means Public-Private-Partnership. Generally large projects are either initiated, funded and implemented by the government or large corporations. That is either they will be in the public sector or in the private sector.
However there are projects important for public service or country's infrastructural development. But either the government by itself or private entities by themselves may not be able to fund and implement fully. So in such cases, considering the need and the utility of the project for the people and development of the nation, a public-private participation is made.
The Kochi international Airport is one fine example of PPP model. Later on Bengaluru and Hyderabad airports are also came by PPP.
PPP has the proper mix of optimum and essential control of the government, but the best sustainable business entrepreneurship also.
Sometimes the PPP may be on BOT basis-Build Operate(for some period. This is resorted to in case of Toll bridges and roads.