"Sats" are short for satoshis, the smallest unit of bitcoin (BTC). They are to bitcoin what pennies are to pounds. But, instead of 100 pennies to the pound, there are 100,000,000 (one hundred million) satoshis in every bitcoin.
🔖 Fun fact: The name sats comes from the creator of bitcoin, Satoshi Nakamoto.
Since one satoshi or sat is equal to 0.00000001 bitcoin (one hundred millionths of a bitcoin), it can be easier to talk about sats than bitcoin for small sums. For example "send me 1000 sats" is easier to understand than 'send me 0.00001000 bitcoin".
If you wish to change your account's bitcoin denomination from bitcoins to sats please check out this guide.