Maplex for ArcGIS first places labels in empty areas on the map. Sometimes, due to space constraints, labels must overlap features. You can control which labels have placement priority and how labels may overlap features by setting weights.

A weight of zero means that the map space used by a feature can still be regarded as available for map labels. A label or feature with a weight of one thousand may not be overlapped. A weight of 600 is an intermediate value that will tend to prevent labels from overlapping the parcel boundaries. If a feature or label must be overlapped by a label, Maplex will choose to overlap the feature or label with the lower weight.