What is modern portfolio theory (MPT)? What is the feasible or attainable set of all possible portfolios?

Question:

What is modern theory (MPT)? What is the feasible or attainable set of all possible portfolios? How is it derived for a given group of investments?
Portfolio
A portfolio is a grouping of financial assets such as stocks, bonds, commodities, currencies and cash equivalents, as well as their fund counterparts, including mutual, exchange-traded and closed funds. A portfolio can also consist of non-publicly...