Base Fee and Priority Fee

Base fee and priority fee are the two components of a transaction fee on Ethereum and networks that copied its fee market design. The base fee is set by the protocol itself and adjusts block by block according to how full recent blocks were; it is burned rather than paid to whoever produces the block. The priority fee, commonly called a tip, is chosen by the sender and goes to the validator as an incentive to include the transaction sooner. Wallets normally display an estimate of both plus a maximum the user is willing to pay, and any difference is refunded. Reading these fields in a block explorer explains why two identical transfers can cost very different amounts at different times.