The “Add Icon” option lets you add an icon to represent a host, service, host group, service group or map. The added icon be used to represent the item on the map, for example a host icon to represent a host.
If you add an icon to represent a host, then all services that depend on this host will also be represented by the icon on the map. The same applies to the host groups and service groups. If you choose to add a host group, then the hosts that are part of this group will also appear when you position the mouse cursor over the icon representing the host group.