A “Maker Fee” is a type of transaction fee that is charged when a participant places an order that is not (fully) matched instantly. This means that the participant is increasing the size of the order book by adding in liquidity. Once the order is fulfilled or matched, then a “Maker Fee” will be charged from the participant.
Michael places a sell order of 1 BTC for $1000. Duke then places a buy order of 1 BTC for $900. Duke’s order will not be matched with Michael’s (considering there are no other participants in the market). In this scenario, Duke’s order is increasing the liquidity of the market as well as the size of the order book. Once Duke’s order is matched in Future, he will be charged a “Maker Fee”.