'Pada' means feet, 'ut' means intense and 'tan' means to stretch, while 'asana' is pose or posture. All put together it means intense leg or foot stretch. Usually, it is referred to as the Prasarita Padottanasana, where 'prasarita' again means to extend. It can be explained as a Standing Forward Bend or Forward Bend Yoga and is one of the fundamental standing postures in Ashtanga Vinyasa Yoga.
The Head to Knee Forward Bend or the Janu Sirsasana is a stretching pose and should not be attempted by you. There is no substitute for the physical guidance of a yoga instructor. But it also helps to read about the poses that you are about to learn. Here is how you can do the Head to Knee Forward Bend.
Parsvottanasana or the Yoga Pyramid Pose is a standing pose, where you bend forward and downward, so that in its most flexible form, your head is almost touching your ankles. It thus provides a great stretch to your hamstrings and lengthens your spine. It also includes proper breathing techniques as your perform the posture.