A multistream (multi) is a group of streams that all appear on the same page with a shared chat, so you and your friends can easily stream together.
Click an icon in the chat and then "Invite to multistream", use the quick settings in the account dropdown at the top right, or in Multistream, type a username and send an invite.
Once you are in a multistream, the invite form is replaced by a note that you are part of one - you can only be in one multi at a time. Everyone's stream shows on the host's page even when offline, and the chat is shared until you leave the multi.
Above four live streams, players switch to the lighter Streamshots view by default. You can disable Performance Mode in the settings or player UI.
The emote picker offers your custom emotes in any multistream you are part of, not just your own chat. See Emotes.