It means “with regard to”.

I'd say a better definition is "as regards" or "with reference to" or (more colloquially) "talking about" or "thinking about", and you can use it with or without "of". For example, "Apropos our meet-up tomorrow, are you able to bring wine?" or "apropos of what you said yesterday, can you ..."

It means “ Relation to”. for example：The story was introduced apropos of nothing.