Sat is short for satoshi. The name is a tribute to Satoshi Nakamoto, the anonymous creator of Bitcoin.
1 bitcoin (BTC) = 100,000,000 sats
Bitcoin works a bit like traditional currency: just as the euro is divided into cents, Bitcoin is divided into satoshis. This allows small amounts to be expressed without using long strings of decimal places.
Conversion examples:
0.00005000 BTC = 5,000 sats
0.00100000 BTC = 100,000 sats
Reading an amount in sats is often simpler and clearer than in BTC, especially for small sums.
Good to know: You may come across this unit in the Bitstack app, for example when viewing certain amounts or transactions.