Question

You know, trash can be useful.

- What do you mean?

(What is the meaning or use of 'You know'? When do we use? What other expressions can we use instead of 'You know'?)

Answer 1

In that question, "You know" is simply introductory. It has no real meaning ... or else it's introducing a question that the question-asker thinks the other person will agree with.

I'd omit it.