Hello @Sarah316,
The coupon is for you to use when making a reservation. Coupons aren't transferable.
The "restrictions apply" section should give the expiration date. They're, usually, valid for a year from the date of issue.
You should have received an email at the time that the coupon was granted. To find the email, you could try using the search function on your email application to search for the coupon code.
You may have received it as a reward for being a superhost.
I found the article below by clicking on "Help" and typing "coupon" in the search box.
How do I use a coupon code?
Steve.