"Stay at home" you hear now everwhere - except in Sweden.
Why is er needed here?
I guess it's because "hålla" means "to keep".. so basically you are saying "Keep yourself at home!"
But I'm not 100% sure so if any moderator or native speakers gets to read it, feel free to correct me!! :D
Er is a plural form of "you" as in "y'all" or "you all" so here, they are inferring that a group of people need to stay home.
Why is "Stanna kvar hemma" wrong?
That's all we gotta do to prevent people from dying. :)
So really stay at home is stay you home?
"Håll" means keep so it is saying "Keep yourself at home"