To understand this ad, you must first understand what a Swedish "tomte" is.
A tomte is sort of like a gnome, who lives on the farm/homestead and watches over the house. We all sincerely believe he exists and we feed him a bowl of porridge on Christmas (at least I grew up doing this).
So while everyone is dancing and celebrating midsummer in traditional folk dress, the tomte has discovered to his horror the modern margarine is being readied for the feast.
This will not do, he declares, and he prepares butter "it's so simple, just churn the cream, add some salt, then manage the butter." I've prepared butter this way myself, and it takes some arm-strength to be honest, but his point is very clear. As the tagline reads: "There are no strange things in butter" (det finns inga konstigheter i smör) So, not strange things, aside from the bearded gnome guy that lives around your house.
Client: Smör