It is possible to cluster your large sets of markers to get more useful information from your map. There is two types of clustering. The first is when you have no group column selected and the second is when you do. It should be noted that at any time you can turn off clustering with the gear icon in the bottom left corner of the map.
No Group Clustering
When you have no group column specified, markers are just group together in close proximity. If you zoom the map in and out, the clusters will change. If you click on a cluster, the map will zoom to fit the markers in the cluster.
When the group column is specified, marker clusters will show a pie chart that represent the group diversity in the cluster. Clicking on a pie chart cluster will show a breakdown of the contained markers. It should be noted that if the group column is all numbers, automatic thematic groups will be created This is useful for things like sales information and demographics. Try clicking on the pie chart cluster below and also zooming the map in and out.