On BitMEX, market makers are those who provide liquidity on the exchange, and market takers are those who take liquidity from the exchange. Liquidity is crucial to exchange as it allows traders to enter and exit the market at any time.
Market makers receive market maker rebates for providing liquidity. You provide liquidity by posting ‘passive orders’. This means that the orders do not fill immediately as they enter the orderbook.
You can achieve passive orders by posting limit orders, stop limit orders, and take profit limit orders. If these orders fill immediately, you will be charged a market taker fee as you have not provided liquidity.
A full trading fee schedule may be found here. A negative fee means that the trader will receive a rebate.
To guarantee that an order receives the market maker rebate, you can use the Post-Only checkbox to ensure that your orders will be cancelled if they were to fill immediately. If you do check the Post-Only option, you run the risk that your order will be cancelled, rather than executed.