UTXO is the unspent output from BTC transactions eg. a transaction that has received BTC and has no linked forward transaction. Spent are referenced with new transactions and previous transaction(s) and concatenates into a new transaction. eg. User A has 1 BTC and needs to transfer 0.5 BTC to User B - Create a new transaction and create 2 transactions  - 0.5 to User B and 0.5 back to User A in the ledger.