Discussions can serve a wide variety of activities in courses. Some basic considerations:
"Asynchronous" means that student messages (posts or replies) are published for others to see in sequence, in an area that remains active for as long as needed. Participants can read others' posts and post their own input at any hour of the day, regardless of who else is also using, viewing, or posting in the Discussion Topic. This is in contrast to "synchronous" discussion tools (like web conferencing via Zoom) where all participants are present at the same time.
As in traditional coursework, you may focus discussion around books, articles, websites, images, video, or other media.
In many campus classrooms, those awkward little chair-desks have the single virtue of being mobile, and easily rearranged for small-group discussions. Similarly, most Learning Management Systems allow you to divide students into smaller groups for discussions.
Most discussion board applications within Learning Management Systems accept file attachments, images, videos, and links to student content posted elsewhere online. D2L also allows students to record short webcam videos. So students can submit their more than just text to a message board, and critique others' work (peer review).
Message boards within LMSes are typically "walled gardens," so that only class members can read or participate within them. So you needn't worry about distracting or inappropriate commentary from the rest of the world, within your online class.
We tend to make comparisons between online, asynchronous discussions and face-to-face classroom discussions, and often that's helpful. But it can also limit our sense of possibilities with the online variety. "I can’t exactly replicate a synchronous seminar-style discussion in an online course," history professor Kevin Gannon explains. "But the thoughtful, deliberative analysis and deep conversation which arise out of asynchronous online discussions can’t be replicated in a face-to-face class, either."