The sidebar contains information you need to know about the chat you're in and its members. As an admin, it's where you manage the chat and for everyone else it's how they turn off notifications, invite someone new to it or leave the chat completely.
If you can't see the sidebar for the chat you're in, slide it out by clicking on the information button in the top right hand corner of the chat window. You can access the sidebar options on mobile the same way.
Here are the things you can do in the sidebar:
Admins can additionally change the title, description and icon as well as remove people from the chat.
Favourite a chat, disable notifications, view activity and sentiment:
Show a list of everyone in the chat, invite new people:
Browse all the media and docs ever shared in it.