Using the app is as simple as accessing the video that you're interested in on the "browser" that comes integrated in the app itself and touching the green arrows that you'll see located on the bottom part of the screen.
At that moment you can select the video quality you want to download. It normally has many different resolution options adapted to the different Android devices compatible with the application.
The downloaded videos will be stored on your SD card by default but you can move them around from one location to another however suits you best.
TubeMate YouTube Downloader is a very easy-to-use video downloader whose main strong point is exactly that--you can have get your hands on any video in seconds.