I read that when we travel into the past past we actually travel to a parallel universe. What is the reason? Who said that?
I haven't heard of travelling into a parallel universe when travelling into the past. But I think you may be referring to a tree like structure of time.
Every action you take, or don't take creates an instant offspring of your current universe.
So if you travel back in time, e.g. 20 minutes back and decide not to write this post, but rather check the worldbuilding wiki and help center, you'd split your universe into to separate and equal universes, one where you actually write this post and one where not. In the path where you do, this results in me sitting here, writing this post or not, on my choice, creating two equal forks of the universe.
But it's all timey wimey and wibbly wobbly stuff, to quote one of the most famous time travellers ever.