UCI stand for Universal Client Identification, or some people call it Unique Client Identification
Either way, It is unique ID number, which CIC assign to each and every applicant.
If you are filing your first application ever, you probably don't have UCI yet. But once the application for Visa, PR visa, or any other petition is filed with CIC, they will issue a UCI number.
I already have a UCI since I am a pre June applicant, with a pending review status. I am planning to reapply. In the Generic Application Form, it also asks for the UCI of my dependants. Should I fill in my UCI in the form of my dependants? I am just confused.