The .apk file installs the app, and is also provided for direct download. To install the app, first on your Android device enable "Unknown sources" in Settings->Security. Then either copy the LJModbusDemo.apk file to your Android device or download it directly from the LJModbusDemo.apk download link. Afterwards, open LJModbusDemo.apk to start installation. After installation you can disable "Unknown Sources".
For full details, including in-depth installation instructions, refer to the download's README.txt file.