An RQ is a residence questionnaire, they send it much less often nowadays ... basically, when they doubt that you have really lived in Canada for the minimum required time to qualify for Citizenship, they flag you and send you an RQ... they ask for all sorts of documents that can proove that you really lived in Canada. Here is a very interesting and complete website about RQ, made by a forum member, eileenf, http://residencequestionnaire.wordpress.com/
Nobody knows what triggers RQ today, but for instance if you didn't declare your travel periods correctly, this can be a trigger for RQ