Your job history is just something every applicant has to provide. If there were gaps in the one you sent in with the application, it is normal for CIC to ask for a new one.
Just do your best to list all your jobs or whatever else you were doing for the time period they asked about. Do not leave any gaps! If you cannot remember the exact date you started or ended a job, give an approximate date (at least the month, if possible), then write an explanation stating you do not remember the exact date and have no way to find it out.
If there were some times when you had no job, you must state what you were doing during that time: studying, being a homemaker, travelling, unemployed, etc. They want a complete list of what you were doing ; if there are any gaps, they will ask you to do the form again.
This is probably something they will not ask about at the interview; or they might ask a few questions about it. It is really more for your background check. At the interview they will probably concentrate more on whether your relationship with your sponsor is genuine.