"A lesson in love."
Translation:En lektion i kärlek.
In English you can use both on and about for "topics", like for instance a book can be either "on" or "about" something (the first is more specific). But we can't use på that way in Swedish, only om.
For lessons, in Swedish they can be either i as in "in a topic", like "a lesson in chemistry", or om as in "about". So en lektion om kärlek is possible (but not as a translation here since it has an exact counterpart in English 'a lesson about love'.