A pyramid scheme is a business model in which the people at the top of the pyramid (i.e., the "founders") reap all the profits, while the people at the bottom (i.e., the "pawns") are left with nothing but empty promises.
Also known as Ponzi schemes, these "investments" are really just cleverly disguised chains of deceit and exploitation.
If it sounds too good to be true, it probably is.
Be wary of any opportunity that promises unusually high returns with unusually little risk, and be especially cautious if the only "guarantees" are "if you just bring in a few more people".
And don't get too excited about "leadership opportunities" or "lucrative compensation plans" - these are often just code for "you're about to get ripped off".