TL;DR from the terms page.
- The promo code will automatically appear at checkout for eligible customers – click APPLY to use it on your order.
- Google One members get a discount of 20%, up to $20. Some qualified users will see a discount of 25% off, up to $25
- This promotion ends on December 31, 2018, or while discounts are available
- Limit 1 discount per Google One plan
- The promo code can’t be combined with other discounts or applied to a past order
- Some brand and product exclusions may apply
Note: I also see the offer on my Google One homepage so that may be another way to check if it is applicable to you.