Ideally, you should ask language-related questions like What does the word "table" mean? or Was bedeutet das Wort "Tisch"? In practice, you can ask all sorts of questions such as Why is the sky blue? or Is the Earth round? :))) ...Of course unless they violate the rules that you can see in Jura's answer...

ask anything you think is ok....

Just don't be a jerk. I swear sometimes. Harm a minor and it is over.

The Italki policy can be seen if you click onto the yellow flag. Be careful of asking questions that fall into the categories below. I think this violates the community guidelines. Chat or rant, adult content, spam, insulting other members, etc. I think this is illegal. Harm to minors, violence or threats, harassment or privacy invasion, impersonation or misrepresentation, fraud or phishing, etc. Here are more guidelines on italki online etiquette http://help.italki.com/Help/GeneralTips