Bitcoin has no accounts and no balances. It has outputs. Each transaction creates outputs, and each output is an amount locked to a condition, usually: whoever can sign with this key may spend it.
UTXO
An output that has not been spent yet is called an unspent transaction output, a UTXO. Every node keeps the full set of UTXOs. Your balance is what your wallet finds when it adds up the UTXOs your keys can spend.
Spending means consuming
A transaction's inputs point to existing UTXOs and carry the signatures that unlock them. Its outputs create new UTXOs for the recipients. Once spent, an output is gone for good: a second transaction trying to spend it is a double spend, and nodes refuse it.
- Inputs: references to UTXOs being spent, plus their signatures.
- Outputs: new amounts, each locked to a new owner's condition.
- Rule: the inputs must be worth at least as much as the outputs.
In the MEMPOOL, each token is a transaction waiting with its inputs and outputs, and its height is the fee it pays.