This is a rehab program for mild to moderate achilles tendon injuries. There will be a lot of stretching, mobility work (lacrosse ball and foam roller) for your calf muscle, hamstrings, quads and hip flexors. You will also strengthen these muscles greatly along the program.
Day 1 is appropriate to start with for anyone (with mild to moderate pain/injury), but other videos progress in dificulty. Listen to your body – it is normal to feel some discomfort but at no point should you feel pain! If the exercises are painful, back off immediately and modify them as instructed.