Before being able to use the Android application, you need to pair the Bluetooth module with your smartphone.
Power the Arduino board, turn on Bluetooth on the Android phone and search for Bluetooth devices nearby: the JY-MCU module will present itself as HC-06, the pairing password is 1234.
The key component of the Bluetooth Arduino Connection App is the Bluetooth Client while the Arduino board will act as Server: this means that the app will always initiate the connection. When you power the Arduino board, the Bluetooth module red LED starts blinking; push the app "Connect to Device" button and select your module from the list:the red LED light becomes solid and the connection status will change to "Connected".
Again, this has nothing to do with Bluetooth Master / Slave, but still can be confusing: you Bluetooth module needs to be Slave but it (or rather the Arduino sketch) will behave as Server in the Client - Server communication with the Android app.
You can install the Bluetooth Arduino Connection App directly from Android Play Store, or you can import the full app code into your MIT App Inventor projects by downloading the attached file.
Bluetooth Arduino Connection App is developed with MIT App Inventor 2; the following step will provide a detailed explanation.