The URL that users will use to access Mattermost. Standard ports, such as 80 and 443, can be omitted, but non-standard ports are required. For example: http://example.com:8065. This setting is required.
Mattermost may be hosted at a subpath. For example: http://example.com:8065/company/mattermost. A restart is required before the server will work correctly.
…is what the System Console is telling about that.
It’s basically the URL which will be “officially” used by anyone accessing your instance. So, it is not related to any private/internal URL, at all. It’s just the public address, visible to every user. Usually something like https://mattermost.domain.tld or something similar.