A host can be considered as a basic grouping unit in Zabbix's configuration. As you might remember, hosts are used to group items, which in turn are basic data acquiring structures. Each host can have any number of items assigned spanning all item types Zabbix agents, simple checks, SNMP, IPMI, and so on. An item can't exist on its own, so hosts are mandatory.
Zabbix does not allow a host to be "left alone", that is, not belonging to any group, since version 1.8. Let's look at what host groups we have currently defined in the frontend, open Configuration | Host groups.
The first thing that catches the eye is that group Templates seems to have a large number of templates already. These are provided as examples, so that you can later quickly reference them for some hints on items. We'll ignore these for now. We can also see an empty Windows servers group, and the Zabbix Servers group which contains a single example host. The interesting part is at the top of the table...