pedagogie

Open your bitcoin wallet. You see a balance, say 0.01 BTC. Your app calculated that number itself. It isn't written anywhere on the blockchain.
What the blockchain $BTC keeps are pieces. Each payment you receive creates a piece in your name, with a fixed amount. Developers call this a UTXO, short for “unspent transaction output.” Put simply: a bill you haven't used yet. Your “balance” is just the sum of all those bills.