Meaning of "Host key"

Remoting in to a terminal server with Remmina, the right-hand Ctrl key is reserved for Remmina. Its function, depending on what one is reading, pertains either to Remmina's "Host Key" or to "Grab[bing] all keys." It's a pain not to be able to use both Ctrl keys, but, before I change the key mapping in Remmina's preferences, what does "Host key" or "grab all keys" mean? What would I be disabling by remapping that key?