Host information

Host starts message:

> :tmi.twitch.tv HOSTTARGET #hosting_channel <channel> [<number-of-viewers>]

Host stops message:

> :tmi.twitch.tv HOSTTARGET #hosting_channel :- [<number-of-viewers>]

what does the start and stop message mean? also, #hosting_channel does that mean the target channel being hosted, or is that the channel initiating the host?