The bid expiry feature allows you to select a time that your bid will expire, preventing the business from accepting your bid (ex. selecting you 1 hour prior to start time). This feature was released to offer Operators the choice of having their bid expire in the event that they need a certain amount of notice prior to bid acceptance. Here's an example of what this looks like:
You have the option of setting your expiration time as early as 24 hours prior to the Op start time and as late as 1 hour before the start time. If you are bidding on an Op and the start time is less than an hour away, you will not be able to set a bid expiry time.