A Satoshi is the smallest unit of a Bitcoin. It is named after Satoshi Nakamoto, the pseudonymous person or group of people who created Bitcoin.
One Bitcoin is divisible into 100,000,000 units, each of which is a Satoshi. Therefore, 1 Satoshi equals 0.00000001 Bitcoin (BTC).
Satoshis offer a way for people to engage in micro-transactions or to buy smaller amounts of Bitcoin, which would be particularly useful if the value of one Bitcoin is very high.
So when someone refers to Satoshis or “Sats,” they’re usually talking about these smaller fractions of a Bitcoin.