Google Play is home to all kinds of apps and games, music and books, and movies and TV shows, and all of it can be purchased with Google Play credits. Usually Google Play credits will cost you the equivalent of how much you want to spend in the store, but right now Amazon is having a sale on digital credits which are delivered to you by email where you can get $50 in Google Play credits for just $45. You do have to enter a code at checkout to get the $5 off and there is no mention on Amazon about how long this deal will last, so you may want to jump on it as soon as possible so you don't miss out.
If you're planning on picking this up, as soon as you head to checkout and are able to enter in a promo code, enter the word GOOGLE into the promo code box, click apply code, and your price will drop down to $45 instead of $50, basically getting you $5 in free Google Play credits to spend on whatever you wish. If the Play Store gift card is for yourself then you only need to enter in your email and name, but if it's for someone else then you will also have the option to add in a personalized message, which could be useful if this is a gift for someone's birthday or another holiday.