It's an American application thing that Canada doesn't use. Essentially, when filling out the AMCAS (similiar to the OMSAS for Ontario, but instead for most US schools) you fill in a portion of your application, GPA, BCPM GPA, extracurricular activities, volunteering etc. You then send this to AMCAS with all of the schools you are interested in; if these same schools are also interested in you, they will send you secondaries. These secondaries are a more in-depth and school specific application. Send that in, then interviews may start rolling in.