The archive contains all the messages sent to a group.
A bounce is an email sent to an address that, for some reason, was not deliverable. This could be because the email address is wrong. Or maybe the person's mailbox was full.
A hashtag is a word or unspaced phrase prefixed with the hash character, #. In Groups.io , you can tag a message with a hashtag by placing that hashtag in the subject of the message. For example, a message with the following subject:
My name is Mark #intro
is tagged with the hashtag #intro
Integration is a nerdy word for a connection to another service. Groups.io allows you to connect your group with other services, like Facebook, Dropbox, Instagram and Github.
A moderator helps run a group. Moderators can have various permissions that let them do different actions within the group.
A group owner is the person that started a group and has complete control over the group. An owner can make other group members Owners as well, or Moderators. A moderator is like an owner, but with perhaps fewer permissions.
A thread is a group of messages with the same subject that form a conversation. Some one sends a new message, and other people reply to that message. That's a thread.