The numbers in the middle of a marker represent the number of sightings within a given area. These numbers and the color of the markers will adjust depending on the zoom level that you are currently set at. Look at the legend to the right for the marker color and the number of sightings associated with that color. The marker colors do NOT determine a level of threat, merely the number of sightings.
When you click on a marker with a number the map will zoom in allowing you to see individual sightings (points). Clicking on a point brings up a window with the address and the date and time of the sighting. When clicking on a numbered marker you may be zoomed in to what looks like a single point. This means that there has been more than one sighting at the given location. When this point is clicked the info window will display only the most recent sighting.
The list of sightings on the left is sorted chronologically with most recent at the top. You can click any of these entries and an info window will pop up on the map pointing to the location.