A real estate Offer is not a Contract until it is "ratified" by all parties. This means that all parties to the contract/offer have agreed to all the terms of an offer to purchase real estate.
Once all the terms in the offer are agreed to, the parties sign and initial the offer and it becomes a contract between the parties. At this point no other offers can be accepted. If there is even one small term that all parties have not agreed to and signed off on, the offer is still just an offer and not a contract yet.
It is best to have your offer fully ratified so that it becomes a contract. If it is not ratified then either party can walk away. If the buyer has not agreed to a term in the offer yet, then the seller would be free to "ratify" or accept another offer on the property.