Ebooks come in a lot of different forms these days, one of which is the .mobi extension. Different apps may or may not open .mobi files though, and it’s hard to tell a smartphone app to “open this file” like you can in windows. The Kindle app by default doesn’t really have a native way to browse for files.
The Kindle app does support these .mobi fils however, and getting it to open them is extremely easy.
All you need to do to get your Kindle app to open a .mobi file is plug your Android phone or tablet into your computer using your USB cable, and open it up to browse the files that are on your SD card. On the root of your SD card you’ll see a folder just called Kindle.
Placing any .mobi file directly into this folder will automatically add the file to your library the next time you open the Kindle app.