An expiry level means the time of termination of a Binary Option. The expiry level shows you the estimated market price of the underlying asset at the very moment when the contract is completed.
You can expect the market price will rise or fall at a certain time. You have the possibility to buy Binary Options for a minute, 10 minutes, 15 minutes, an hour, a day, a week and a month. So if you buy Binary Options you know how much time is left until the point of expiration.