This app is designed with one intent and one intent alone: to save the audio from videos hosted on YouTube to your device. If you want to download a full video or anything that includes the images from the video, you'll need to find another application.
Once you've chosen the video, just copy the URL and paste it into the bar in the app to download it. You can also choose the format in which you want the final result to be saved. Either use the original format or choose another that better suits your needs. Download all the music you want with FreeGrabApp Free YouTube To MP3 Converter.