Based on point locations and the number of points, the tool creates a fishnet mesh to overlay your points. After counting the number of points that fall within each fishnet square and removing squares that are outside your bounding analysis areas, fewer than 30 fishnet squares were left. This tool requires a minimum of 30 counts (30 squares) to provide reliable results.
If your points are located at a variety of locations inside the bounding analysis areas, you may need to provide larger polygon boundaries. If your points occupy very few unique locations (if there are many coincident points), a good solution is to provide aggregation areas that overlay your points.
Another option is to specify an analysis field. However, this changes the question from where are there many or few points to where do high and low analysis field values spatially cluster.