I agree with everything @Glorfindel said, but have slightly different emphasis and extra details.
It can be quite easy to tell that a question is not good or even quite off-topic even if you have no idea of a good answer or of what a good answer would look like. This is the positive side of reviewing.
Extreme cases include spam and posts with offensive content. It is often better just to flag the question for moderator action. But this is something you grow into. It's not always out of order that several people register displeasure, at least briefly, to send a strong signal, moderately expressed, to the OP. Much depends. There is a risk that you get caught up in cross-fire, which does not help anybody, and can just lead to frayed tempers and a stressful exchange. On the other hand, some offending posters regard moderator action as arbitrary abuse of power by one person, and commenting in support of a moderator -- or in anticipation of a likely moderator action -- does signal a shared reaction to inappropriate content. But if in doubt, just flag.
A comment can always express a measure of diffidence, as in "Not my field at all, but my guess is that even experts in this need much more information". That way, it should be clearer that you won't be following up with much more.
A common reaction to "Not clear" is "How do I make it clear then?" which can be difficult to answer to satisfy the OP. If you don't know what is being asked, then it's likely that you can't suggest a way to rewrite it. But advice
- to be more specific
- to show some data or more about them, giving a graph or summary statistics
- to give an example
- to explain the goals of a project
- to spell out what "didn't work"
- to give a context
is often easy. Many posters are naive, or lazy, or both -- or they expect a first post to be just the start of an exchange. Treating a forum as if it were a helpline is part of what can be going on here, and it can be sensible too. If I consult a medic I expect to show a problem or report symptoms, but then in turn to be asked several questions by the medic.
The riposte "It's clear to me, so why isn't it clear to you?" is also common and hard to answer constructively.
Posting comments always carries some obligation to keep an eye on the thread and to delete them afterwards if they become obsolete, or someone else posts a better comment, or the comment turns out to be irrelevant, and so on.