UTXO stands for Unspent Transaction Output. Since Bitcoin, forks of Bitcoin and other altcoins don’t store account information like balance, they instead determine how many Bitcoins can be sent as outputs by taking all previous outputs from all previous inputs. Whatever remains must be the balance. When a wallet tells you that you have 5 BTC, it has actually analyzed the UTXO information of the entire history of the chain to determine this value.