bedroom tax - please explain

I don't understand what or who the bedroom tax is aimed at. I have three children, the oldest who is four years old, is autistic and we are looking at getting a four bedroom house. Will this tax mean that children can't each have their own bedroom or does it just mean people who have empty spare bedrooms that aren't in daily use will be targeted by the bedroom tax?

Is anyone able to explain this tax please?

My two oldest children are boys and my youngest is a girl. My oldest son isn't able to share a bedroom with his brother as he physically attacks him to the point of drawing blood and causing his younger brother serious injury. At the moment the younger son and daughter share but I would like them to each have their own rooms.

I just don't understand what the exact aim/purpose of this tax is.