They're referring to the cryptocurrency USDT (US dollar tether) which is always supposed to be worth 1 US dollar. They mean that you should sell your BTC in exchange for USDT, to avoid having to go through the complex processes and taxes involved in selling for fiat directly.
However, this appears to be a
bad idea. USDT appears to be
spawning money out of thin air and potentially even manipulating the market with it.
It's not at all certain that you will be able to exchange your USDT for fiat when you want to do that.
