where are the free books the kindle app offers?

According to [its description on Google Play](https://play.google.com/store/apps/details?hl=gb&id=com.amazon.kindle) , in the third sentence, it says that the books are free. I installed the *Kindle app* and went to the *Kindle Store* and all the books I looked at so far all have to be purchased. Where can the free ebooks be downloaded from? Because when I click 'books on device' in the kindle app, it shows none (so I'm guessing I need to download them from somewhere).