Within a group, there are usually many discussion topics. The Default View Topics permission allow you to define the default level of view access for users according to their role, which can be owner, manager, member, or anyone:

In some cases, you might want to restrict access to the topics lists of a channel. Owners only will make it so that only the owners of the channel can see the topics list, while using Owners and managers will also allow managers to see it. All members of the group will only allow members to read the full topics list, and Anyone on the internet will open it up to the general public.