Discussing with university professors before being undergraduate I have applied for a computer science course at more universities, but one of them really convinced me with a very welcoming attitude. Even if I have not confirmed my place, I have a tutor and he introduced me to some professors in the university to discuss about some topics that I am interested in. But, I find myself in a rather delicate situation. I have just some basic knowledge in their subject, so a discussion with them wouldn't be so entertaining for them, I guess.

Of course, there are a lot of things that I would like to learn, and so many questions to ask, and they seem willingly to answer (actually, ever they do not know that, I will go to their university), so, to what extent may I ask them?
Most professors will enjoy talking about their subject with a student who is motivated and interested, so you shouldn't be bashful about asking them questions. Just try to ask "good" questions, as opposed to questions which can be easily answered with a moment's search on Wikipedia.