The expiration date of a bonus is the deadline to claim or complete it. Every bonus has specific terms, including an expiration date — if you don't use it before then, the bonus funds are lost.
Check your active offers at My Account → Promotions → Active Promotions. If a bonus you're expecting isn't there, check the source it came from — an email, a personal message, or the Offers section under the 🎁 icon.