Host as in the altar bread that is used in the Eucharist comes from the Latin Hostia which means a sacrficial victim. As Jesus is our sacrificial victim on the cross, and as the host will by God’s power become the real body of Jesus at the Mass, we refer to the altar breads as hosts.
As to Heavenly Hosts or Lord of Hosts. It is an english translation of the Biblical reference to a large number of followers, or in the case of Heavenly Hosts, the Angelic army of God against the army of Satan. So in this case hosts would be a large gathering.