What is a Satoshi?
A satoshi is the smallest unit of Bitcoin, named after its pseudonymous creator Satoshi Nakamoto. One Bitcoin equals exactly 100,000,000 satoshis--think of satoshis as "Bitcoin cents," but much smaller.
Why Stack Sats?
As Bitcoin's price rises, satoshis become the practical unit for everyday transactions. Many Bitcoiners "stack sats"--accumulating small amounts regularly--embracing the long-term sound money perspective.
The Conversion Math
To convert satoshis to USD: multiply satoshis by the current BTC price, then divide by 100 million. At $100,000 per BTC, one satoshi equals $0.001 USD.