Quite self explanatory. Heaven only knows why the man is wearing a hat and sunglasses indoors. Bizarre. Anyway, the exercise is good. Try to keep the ball of the foot reasonably in contact with the floor – no more lift than is in the video. And, as he says, keep your feet pointing forwards.
Not everybody has a weak tibialis posterior, so it won’t help everybody. I personally would start the exercise slightly differently, especially if, like me, you have a weakness here. Otherwise you could strain your calf muscles. Good luck.