To top it off means to add to it, to fill sometime up completely. To the top.
If I have a drink that is half full. I'll ask someone to 'top it off' for me.
It is sometimes used as a metaphor like "To top it off, my manager made me dust the shelves." As if, your list of things to do was already filling up. And then someone adds one more thing to fill it up to the top.