I understand it but can't put it exactly. So I am quoting.

{Source : Google}

HYIP is High Yield Investment Program. People can invest money into high yield, high risk markets who couldn't normally do so on their own, due to lack of initial investment funds. A HYIP pools money from investors together and uses the funds to invest in these markets; investors receive a portion of the return on their investments over a pre-designated amount of time. Most HYIPs pay interest daily, weekly, or monthly over the Internet. While some HYIPs are valid investment tools, a majority are actually one form of a ponzi scheme.

{Source : Google}