I think the window is a metaphor for her relationship with the man she's singing about. She feels that the relationship has insulated her from the outside world, made everything outside unclear. She needed to break it - to break up the relationship - in order to see the world clearly again. The "her" she refers to is probably a woman he was cheating with. The window was blocking her view of that cheating. "When it's clean it's so clear that I can't tell what I'm looking through" means that sometimes, she feels like she does perceive the outside world, but at those times, she doesn't feel like there's a relationship.
I think the window is a metaphor for her relationship with the man she's singing about. She feels that the relationship has insulated her from the outside world, made everything outside unclear. She needed to break it - to break up the relationship - in order to see the world clearly again. The "her" she refers to is probably a woman he was cheating with. The window was blocking her view of that cheating. "When it's clean it's so clear that I can't tell what I'm looking through" means that sometimes, she feels like she does perceive the outside world, but at those times, she doesn't feel like there's a relationship.