There are many variations on this theme, but they essentially involve the promise of crypto rewards for "investing" hard cash into an online mining platform.
Note that there are legitimate mining pools where you rent out your computing power in exchange for a share of mined coins. But the scams are far more common.
A classic example: you pay a minimum "investment" of $500, receive regular updates showing earnings, and get phone calls encouraging you to invest more. When you try to withdraw, you are told to pay a variety of fees before funds can be released. You will never receive any rewards at all.
An alternative variation allows you to sign up for free, then bombards you with calls to pay for upgrades promising more BTC and quicker withdrawals. Needless to say, you will never receive any BTC. Not ever.
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