On your home screen where it tells you how many books you’ve read on the bar, go down and it also shows the badges you’ve earnt and at the bottom there is a space to type in your code and enter it. You can’t enter it more than once.

I have no idea either

If you write book reviews that are interesting and informative, you could find yourself being awarded the Star Reviewer badge. The badge will show up on your profile and next to your username in Chat and on book pages.

I found this on the internet hope it helps.
If you want to find out more search on google : How do you get the star reviewer badge on the summer reading challenge, then click on the first link.

Princess Pixie Washington