Upward Facing Dog is a great chest opener and back stretch. A lot of times it is just called "up dog" for short. This posture is important because it connects to Downward Facing Dog and many other flows or Vinyasa.
Shine your heart forward and keep your gaze looking forward. Try not to overbend your lower back and over compress your lower vertebra. Keep your fingers spread apart wide and strong. Squeeze your buttocks and engage your quads. This will slightly lift your knees off the mat which is what you are going for. You can rest in this posture as long as you like or connect it to Downward Facing Dog.
To connect the two postures, inhale into an Upward Facing Dog, then exhale into a Downward Facing Dog. Meddy Teddy loves to connect these two dogs together.