I am watching this video on youtube. Is the cat trying to get close to the baby or is it some kind of aggressive behaviour? By the way, the cat in the video is a female cat and gave birth to three kittens last year.
No, the cat is just wary of the baby, but also very curious. Initially, the cat was tense and looked like it might flee. Then you can see its body relax. Next, it starts investigating, gradually getting closer to the baby. Finally it decides the baby is probably not a threat, and sits near it, but at a safe distance. As the cat sits, its body relaxes, but it's still a bit wary.
Compare this with how a cat looks when its chasing prey. The cat crouches very low to the ground (even lower than in the video with the baby), it shifts from side to side to get the right balance, the tail flicks back and forth, and then... POUNCE.