Overt racism is like a formal declaration of war whereas passive or ingrained racism is like fighting the War on Terror. Overt racism has a face and can be attributed to that one person's or small group's upbringing and life experience. It's something you can lash out at and break the teeth of, or report to the police or the community at large. Passive racism has no face. You can try to fight it, but if no one else sees it then suddenly you're crazy, or overly sensitive, or making mountains out of molehills.
For me, one clear example is the difference in the way people of color are treated if they speak with an accent vs. how white people are treated if they speak with an accent. On some level, it's a psychological trigger that lets the listener know that the speaker is not from wherever "here" is. The almost automatic assumption for most people would be that the white with an accent must be intelligent, because they speak more than one language. The automatic assumption for the person of color is that they don't speak English correctly.