In AMM-based decentralised exchanges a protocol fee is the fee
taken from your trading costs that goes towards the protocol treasury.
Protocol fee is revenue for the protocol itself.
The distribution of the protocol fee is decided by a DAO.
Usually protocol fee is 0.00% (Uniswap v3 default) - 0.005% (PancakeSwap, Sushi).
See also
Swap fee
Decentralised exchange