Average fee paid per transaction size, in Satoshi/byte
Bitcoin transaction fees are paid to miners who confirm transactions by including them in a mined block.
Because blocksize is limited, miners typically assign priority to transactions with higher fee rate. Recently, demand for on-chain bitcoin transactions has become so high that transactions without a fee are almost guaranteed to be ignored by miners, and the average fee required for a transaction to be confirmed has increased.
For most users, Bitcoin transactions are created by wallets that choose the fee rate. Some services offer fee rate estimates for how soon a transaction with the given fee will be included in a block.
Bitcoin transaction fees are measured in Satoshis per byte (0.00000001 bitcoin per 0.000001 Megabyte) because memory space on the blockchain is the limiting factor. Several factors may influence the size of a bitcoin transaction in bytes, however the value of the transaction in bitcoin is not usually correlated (a typical transaction sending 1 bitcoin is usually about the same size in bytes as a typical transaction sending 100 bitcoin, and will need to have about the same fee to receive the same confirmation priority).