Orderbook is a book with open orders, that waits for a counterparty to handshake and sign the contract. When two opposite orders meet, the settlement occurs and the contract is signed.
At any moment you can place how many orders you want and also cancel them because it happens not on the blockchain but through meta transactions. This is why it’s will be free
Meta transactions are like signed bank checks, which are requests with your crypto signature. If two opposite requests with the right crypto signature meet on the blockchain, then a contract is signed.