A draft message is automatically created any time you click the "New Topic" button, or hit "Reply" to a message online. If you subsequently click away, or otherwise leave the message editor -- even if you never typed anything -- a new menu item "Drafts" will appear in the left-side menu bar for that group. The draft will contain any edits you may have done to that message in the editor up to that point.
Clicking open the Drafts menu opens a page that looks much like the provided screenshot. Here you can open the message, complete your edits and send, or simply discard.
In addition, if you send an EMAIL message (i.e. not online), using a restricted hashtag or some other prohibited construct, the offending message will be held in Draft form until you log into the group and fix it. Groups.io automatically sends you an email explaining why your message did not post, containing a link to the draft.
You can have several draft messages active at the same time (see screenshot at right). Each draft will persist until you eventually send it as a message or discard it.