At first blush it might seem like an innocent inquiry about how to focus one's study efforts. But the very clear implication is that those students asking the question won't bother studying things that they know won't be on the test. That presumes there are just a few things in a course that you really need to know, that the tests will cover those few things and nothing else, and hence that the purpose of the course is to train you to master those few things. In our modern age of rampant standardized proficiency testing, it's common (though regretable) for many primary and secondary school instructors to "teach to the test" like this. So I guess it's understandable (though regretable) that many students will come to expect more of the same at college.
But in a more mature environment, that line of thinking is wrong -- very dangerously wrong. There's far more that you're expected to know and learn in a course than can possibly be checked on a couple in-class exams. Think about an exam as a represenatative sample of what you're expected to know and learn and do. That sample will certainly include most of the highlights, but will also include a smattering of the smaller, lesser concepts and ideas that you're expected to know as well. The students who have a more thorough understanding of this overall range of ideas will be the ones who stand a better chance on the few things that actually appear on the exam.
So just because there are many things that simply won't all fit in a one-hour snapshot of what you've internalized from several weeks of course experience, it doesn't follow that most of the material in a course is dross, and that there are just a few tidbits that the prof actually wants you to know. This couldn't be further from the truth. Most profs do so much culling and hashing and sifting and distilling, and they still have to cut out most of what they'd like to do in any given course. What they actually do in class is what remains from this narrowing process, so you can bet that everything you see in class is something they've chosen to include at the expense of many other ideas they would like to have included as well. It's all to be considered worthwhile, even though it all can't possibly be checked in a couple puny exams.
So try to avoid The Question That All Profs Dread and its cousins. It might seem harmless, but only to students who harbor some dangerously naive notions about the nature of education and the role of testing, and about how most profs create courses. Even if they're not aware of it, students who want to know exactly what few things will actually appear on the test are essentially asking what not to study. Not only is it an unfair and insulting question, but it exposes you as a student with a junior-high approach to your own education.