When you publish an ebook on Amazon through KDP, you have the option of enabling DRM, which stands for digital rights management. This ostensibly makes it harder for bad guys to pirate your ebook. I say ostensibly because it is, in fact, very easy to crack the Amazon DRM if one is so inclined. On the other hand, DRM can frustrate people who have honestly paid money for your ebook but want to read it on a non-Kindle device or in a non-Kindle app.
I’m not Stephen King or JK Rowling, so I’m not too worried about people pirating my books. At the end of the day, it probably won’t matter a whole lot for you, either, whether you enable or disable DRM. Either way, it will only affect a small percentage of the people reading your book. I personally never enable DRM on any of my books, because I don’t want to give people any reason to not like my books.