Marker clustering allows for the grouping of multiple map markers into a single, composite marker allowing for the visual drill-down into a group of closely positioned markers.
The settings related to this feature are shown here:
Cluster Map Markers: This enables and disables the map marker clustering system.
Cluster Zoom: This is the zoom level that the map will stop clustering markers. Maps on Google Maps have an integer "zoom level" which defines the resolution of the current view. Zoom levels between 0 (the lowest zoom level, in which the entire world can be seen on one map) to 21+ (down to individual buildings) are possible within the default roadmap maps view.
Cluster Grid Size: This is the pixel area used by the marker clusterer to group markers. Larger grid sizes group more markers over a wider area. Smaller grid sizes will make smaller groups but also take more processing when loading the map.
Advanced detail regarding Google's Marker Clustering and other marker management techniques can be found here.