In this case, 'something' works as a pronoun. It means it's substituting for whatever it is you need an opinion on. Example: "I need your opinion on [the article I wrote]." You could replace [the article I wrote] with a pronoun, such as "this", "that", "something", etc.. The difference with "something" is that it's undetermined/unspecified/vague (unlike "this" and "that"). When you say "I need your opinion on [something], it means you're waiting for the other person to agree to your request before you tell her what it is you want an opinion on. You could see it as a softer/politer way of asking someone for their opinion.

This sentence is used to get someone's attention before telling him/her what you need an opinion on. Person A: "I need your opinion on something." Person B: "Alright, what is it?" Person A: ".. this .. that .. "