A host is a computer that uses virtualization software to run virtual machines. Typically, a host is a computer running ESX, XEN or Windows 2008 Server software.
Hosts provide the CPU and memory resources that the virtual machines use and give the virtual machines access to storage and networks. Multiple virtual machines can operate on the same host at the same time. Adding a host to themangement server allows you to manage it and the virtual machines that run on it.