UTXO (Unspent Transaction Output): The individual "chunks" of Bitcoin that haven't been spent yet. Your Bitcoin balance is the total of all these chunks. The global UTXO Set acts as the ultimate ledger, by adding up every unspent chunk in existence, the network can prove exactly how many Bitcoins are in circulation at any given second.

An Analogy - "Loose Change"

In a normal bank account, you have a single "balance" (e.g., £500). When you spend £10, the bank just changes that number to £490.

Bitcoin doesn't do that. Bitcoin doesn't have "accounts" or "balances." It has UTXOs, which are like individual banknotes or coins sitting in your wallet.

The Scenario: Imagine you want to buy a sandwich for £7, but in your physical wallet, you only have a £10 bill.
The Transaction: You can’t tear the £10 bill in half. You have to give the whole £10 to the cashier.
The Result: The cashier takes the £10 and gives you back a £3 bill (change).
The UTXO: That £3 is an "Unspent Transaction Output." It is a new "piece" of money that is now sitting in your wallet, ready to be spent later.

In Bitcoin: Your "balance" is simply the sum of all these different-sized "pieces" of Bitcoin you haven't spent yet.

2. How it Determines "Total Supply"

This is the "Accounting" part of the UTXO model. Every single node (computer) on the Bitcoin network keeps a list of every single "unspent bill" in existence. This list is called the UTXO Set.

Verification: When you try to send Bitcoin, the network checks the UTXO Set to make sure you actually have those "unspent bills" to give.
The Total Supply: To find out exactly how many Bitcoins exist right now, the network doesn't look at a master "total" number. Instead, it adds up the value of every single unspent piece of Bitcoin in the world.
The Calculation:
$$\text{Total Supply} = \text{Sum of all UTXOs in the global UTXO Set}$$

3. Why it Matters (The Benefits)

Auditability: Because every "bill" has a history, you can trace every single Satoshi (the smallest unit of Bitcoin) all the way back to the moment it was "mined" (created).
Security: It prevents "Double Spending." Once you spend a "bill," it is removed from the UTXO Set and replaced with new ones. You can't spend the same bill twice because it no longer exists as "unspent."

https://www.dbm.academy/faq for more blockchain vocabulary.

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