"Sats" is short for "satoshis", which is the smallest unit of Bitcoin. It's named after Satoshi Nakamoto, the pseudonymous person or group of people who developed Bitcoin.
One Bitcoin (BTC) is equivalent to 100 million Satoshis. Therefore, satoshis represent a fractional part of a bitcoin and are to bitcoin what cents are to the dollar or euro, or pence are to the pound. This allows people to transact in smaller units of the currency, which is useful as the price of Bitcoin continues to rise.
So, if you hear people talking about "sats", they're referring to this smaller unit of Bitcoin. For instance, instead of saying "0.00005 BTC", you could say "5,000 sats".