You pressed send four hours ago, and the money hasn't arrived. There's no one processing your request, no customer service, and most importantly, no queue. What exists is an auction for space, and your transaction simply didn't bid enough.
Each network node stores valid transactions that haven't yet been included in a block in its memory. This deposit is called the mempool, and it's important to understand that it's local: your node has its own, mine has mine, and the two are almost never identical. There is no official waiting list anywhere in the world.
A block can practically fit just over a million size units, which amounts to between two and three thousand transactions. Since space is fixed and demand is not, the miner who assembles the next block chooses the transactions that pay the most per unit of occupied space. That's why the fee is measured in satoshis per vByte, not in reais or as a percentage.
This leads to the most counterintuitive fact about the subject: the fee doesn't depend on the amount sent. Sending a thousand bitcoins in a compact transaction can cost less than sending half a real in a transaction with thirty inputs, because what you pay for is the space occupied in the block, just like freight charges by the volume of the cargo and not by its price.
The price of this space varies greatly. In December 2017, with the network congested, the average transaction fee exceeded fifty dollars. In block 840,000, mined on April 20, 2024, miners collected more than 37 bitcoins just in fees — a record caused by a rush to record data in that specific block. On quiet nights, the same transaction costs only a few cents. Your wallet checks recent blocks and the size of the mempool to suggest a value, and it gets it right most of the time.
When it gets it wrong, there are two solutions. The first is to replace the transaction with an identical one with a higher fee. This mechanism is called RBF, or replace-by-fee, and it works because the stalled transaction is not yet definitive: while it's only in the mempool, a new version that spends the same inputs and pays more is accepted in place of the old one.
The second solution is for when replacement isn't possible — because the transaction was created without this permission, or because you're the one waiting, on the receiving end. It's called CPFP, child pays for parent. You spend the unconfirmed output in a second transaction, paying a high fee on it. Since the second depends on the first, the miner can only earn that hefty fee if they include both together, and that's what they do.
And if you do nothing? Nothing bad happens. The coins remain yours because an unconfirmed transaction hasn't moved anything — it's just a proposal. After a while, by default two weeks, the nodes discard the proposal from memory due to lack of space, and it's as if it never happened. The pieces it was going to spend remain intact, available for a new transaction.
There's only one way to lose money in this story, and it's the usual one: accepting something as paid that hasn't yet been confirmed. A transaction in the mempool can be replaced, forgotten, or never mined. Confirmation is depth, and the beginner's trail has already explained why.
What's left is to see what happens on the other side when the miner finally chooses the transactions and assembles the block. It's eighty bytes of header that summarize everything, and a number that varies billions of times per second. In the next lesson, the block from the inside.