The first point to understand is what a marketable order is.  Marketable orders are either market orders or buy and sell limit orders whose limit price is at or above/below the current market price.  A marketable buy limit order would have a limit price set at or above the current ask in the market.  Conversely, a marketable sell limit order would have a limit price set at or below the current bid in the market. 

Marketable orders remove liquidity from the market. 

Non-marketable orders are buy and sell limit orders in which the limit price is below/above the current market price.  A non-marketable buy limit order would have a limit price that is below the current ask in the market.  Conversely, a non-marketable sell limit order would have a limit price that is above the current bid in the market. 

Non-marketable orders add liquidity to the market.