Classically it's things that do not benefit the practice. That's the big general term. Things that do not actually benefit the practice, he wasn't that interested in getting involved in. Specifically, it was often when people asked about the beginning of time, the end of time, where do Buddhas go when they die, all sorts of metaphysical questions. He would simply stay out of those sorts of questions.
Even if he knew some of the answers, there were many times that he wouldn't say it because he knew it would not benefit the person who was asking the question. And on some of the questions he even admitted that he didn't know the answers, such as the beginning of time. It's recorded he couldn't see the beginning of his first birth. So there were some things that he simply did not know and there were other things he did know but wouldn't comment on because he did not feel it would benefit the listener.