The HTC One M9 hasn't been selling particularly well. Certainly not up to HTC's expectations, at least. To encourage a few more people to pull the trigger, HTC is offering some free Google Play credit. Specifically, $100 of it.
To get all that free Google Play money, you just have to buy an HTC One M9 via HTC's website (any version). Keep in mind the full price of that phone is usually $650. Far be it from me to tell you whether or not that's worth it.
After buying the M9, you just have to go to the promo site and redeem the offer using the IMEI and a copy of your receipt. The Google Play credit can be used on music, movies, books, and apps in the Play Store. You have to buy before August 31st to get the credit.