A double-entry ledger is a ledger in which each transaction is recorded as at least two entries of equal value: a debit to one account and a credit to another. Because the two sides always match, total debits equal total credits, and money cannot appear or vanish without a trace. The method underlies every company's general ledger.
Payment companies, wallets and banking platforms use the same principle in their software to track customer money. A transfer between two users is stored as a debit to the sender's account and a credit to the recipient's; a fee is a debit to the customer and a credit to a revenue account. Entries are never edited or deleted, only reversed by new entries, which provides a built-in audit trail. Such a ledger, often exposed to developers through a ledger API, is the source of truth for balances.