This is my understanding, however it might be incorrect, I trade options mostly.
Adding liquidity is when you use a limit order that doesn't execute an order. I.e. bid ask at 57/59 and you but in a limit order to buy or sell at 58. Your order doesn't hit another immediately, so it sits until someone takes it.
A market order will always remove liquidity because you are taking the closest bid (if selling) or ask (if buying), therefore removing liquidity.