A “Sat” or "Sats" are short for Satoshis, the smallest unit of Bitcoin (BTC). One Satoshi (sat) is equal to 0.00000001 BTC (one hundred millionth of a Bitcoin). In other words, there are 100,000,000 satoshis in a Bitcoin. The name comes from the creator of Bitcoin, Satoshi Nakamoto.
If you wish to change your account's Bitcoin denomination please check out this guide.