Connect android mobile:
There is always a need to test the app for proper functioning so for that in Android Studio . There are two ways to test the app first , Run the app on Android Emulator or System device and another way is to run the app on your own android mobile.
For Android Mobile , we just need to connect the android mobile via USB in computer. After connecting mobile , your device will show in the below dialog box of Android Studio .If not appear in it , Enable Intel HAXM in your BIOS settings.
Select the device (if shown) from dialog box to run the app, it wull do gradle in Android Studio and will send APK to target device for launching the app.
Problem will be discussed in Comment section.