Post only order is a type of order that will appear in the order book only if it is not matching an existing order on the opposite side. It is usually used to place an order at the value closest to the current market price, but avoids having to pay the taker fee because it is matched instantly, except if matching with an hidden order (rule may depend on exchange).
Pros: Nearly a market order without paying taker fees.
Cons: Can be difficult to place because of instant cancellation on liquid markets.