I would assume it means just don't slack off too hard. You definitely need to at least make sure your term and overall GPA is acceptable (though how do you define "acceptable"). Admissions is conditional, and they do require your final transcripts prior to matriculation.
There are people who get accepted December 1, and then end up failing an outstanding prerequisite course (usually physics, since that is what most pre dentals tend to put off until their last year) in the winter semester. That kind of thing can really get you in trouble. For schools like Michigan, where they start in June so you cannot retake the course during the first half of summer, you have no chance to make up for it. Otherwise, I don't think a few more B's than usual will be the deal breaker.