It means that your resource is overallocated. In order to remove the overallocation, you should level by selecting
Tools | Level Resources...
and then, next to "Look for overallocations on a " select Day-by-day.
MS Levelling is performed by pushing out tasks, not by pulling them closer to the start date if a gap in the resource assignments is apparent.
There are elements which Levelling looks at including things like constraint types and constraint dates, task priority, resource availability calendars, etc.