The date on which an offer will expire. Until that date, the seller has the right to accept the offer, which creates a binding contract. After that date, the buyer has the right to approve the contract before it is binding. In other words, if the seller signs the offer after the expiration date, it is a counteroffer subject to buyer's approval.
In addition, any counteroffer by the seller terminates the offer. So in most instances the Offer Expiration Date is irrelevant, because in most instances a seller will counter on price if nothing else.