A block has a size limit: 4 million weight units, roughly 1 million virtual bytes. Miners have this limited space to sell every ten minutes, and they fill it with the transactions that pay them the most for the space they take.
Price per byte, not per amount
A payment of 1 BTC and a payment of 1,000 sats can take the same space. So fees are compared per unit of size: satoshis per virtual byte, written sat/vB. A simple transaction is around 140 vB; at 10 sat/vB it pays about 1,400 sats, whatever the amount sent.
When many people want to transact, the price of space rises. When the mempool empties, a few sats per vbyte is enough. Wallets estimate the rate needed to get into the next block, the next hour, or the next day.
Stuck? Two ways out
- Replace by fee (RBF): the sender broadcasts a new version of the transaction paying a higher fee.
- Child pays for parent (CPFP): the receiver spends the unconfirmed output with a high fee, so a miner must include both to collect it.
In the MEMPOOL tank, height is the fee rate: the sieve shows the price a transaction needs to make the next block.