It's when your Member of Parliament (usually their staff) calls someone at CIC and asks about a case. It typically doesn't do anything but get a brief update - ie, "they're doing background check."
Getting an MP involved is potentially a good idea when there is an irregularity around a file, if there is a misinterpretation, etc. I would not recommend it if your application is just taking a while (ie, still within or close to the 12 month target) or if you just haven't received a specific stage of the process by when "everyone else" seems to.